diff --git a/code/mission/missionparse.cpp b/code/mission/missionparse.cpp
index c794edf96cd..0e0d9e62dd6 100644
--- a/code/mission/missionparse.cpp
+++ b/code/mission/missionparse.cpp
@@ -1262,6 +1262,8 @@ void parse_player_info2(mission *pm)
 
 void parse_cutscenes(mission *pm) 
 {
+	Assertion(Locked_sexp_true >= 0 && Locked_sexp_false >= 0, "SEXPs are not yet initialized!");
+
 	if (optional_string("#Cutscenes"))
 	{
 		mission_cutscene scene;
@@ -1564,6 +1566,8 @@ void parse_music(mission *pm, int flags)
  */
 void parse_fiction(mission * /*pm*/)
 {
+	Assertion(Locked_sexp_true >= 0 && Locked_sexp_false >= 0, "SEXPs are not yet initialized!");
+
 	if (optional_string("#Fiction Viewer"))
 	{
 		bool fiction_viewer_loaded = false;
@@ -1665,6 +1669,8 @@ void parse_cmd_briefs(mission *pm)
  */
 void parse_briefing(mission * /*pm*/, int flags)
 {
+	Assertion(Locked_sexp_true >= 0 && Locked_sexp_false >= 0, "SEXPs are not yet initialized!");
+
 	int nt, i, j, stage_num = 0, icon_num = 0;
 	brief_stage *bs;
 	brief_icon *bi;
@@ -3214,6 +3220,8 @@ extern int parse_warp_params(const WarpParams *inherit_from, WarpDirection direc
  */
 int parse_object(mission *pm, int  /*flag*/, p_object *p_objp)
 {
+	Assertion(Locked_sexp_true >= 0 && Locked_sexp_false >= 0, "SEXPs are not yet initialized!");
+
 	int	i;
     char name[NAME_LENGTH];
 	ship_info *sip;
@@ -7413,13 +7421,18 @@ bool sexp_is_locked_false(int node)
 {
 	// dunno why these are different, but they are
 	if (Fred_running)
+	{
+		Assertion(Locked_sexp_true >= 0 && Locked_sexp_false >= 0, "SEXPs are not yet initialized!");
 		return (node == Locked_sexp_false);
+	}
 	else
 		return (Sexp_nodes[node].value == SEXP_KNOWN_FALSE);
 }
 
 void set_cue_to_false(int *cue)
 {
+	Assertion(Locked_sexp_true >= 0 && Locked_sexp_false >= 0, "SEXPs are not yet initialized!");
+
 	free_sexp2(*cue);
 	*cue = Locked_sexp_false;
 }
@@ -8926,6 +8939,8 @@ int get_warp_in_pos(vec3d *pos, object *objp, float x, float y, float z)
  */
 void mission_bring_in_support_ship( object *requester_objp )
 {
+	Assertion(Locked_sexp_true >= 0 && Locked_sexp_false >= 0, "SEXPs are not yet initialized!");
+
 	vec3d center, warp_in_pos;
 	p_object *pobj;
 	ship *requester_shipp;
diff --git a/code/missioneditor/missionsave.cpp b/code/missioneditor/missionsave.cpp
index 39e00a2d078..84761254254 100644
--- a/code/missioneditor/missionsave.cpp
+++ b/code/missioneditor/missionsave.cpp
@@ -2134,6 +2134,8 @@ int Fred_mission_save::save_events()
 
 int Fred_mission_save::save_fiction()
 {
+	Assertion(Locked_sexp_true >= 0 && Locked_sexp_false >= 0, "SEXPs are not yet initialized!");
+
 	if (mission_has_fiction()) {
 		if (save_config.save_format != MissionFormat::RETAIL) {
 			if (optional_string_fred("#Fiction Viewer"))
diff --git a/code/parse/sexp.cpp b/code/parse/sexp.cpp
index 7b75b5d4ad4..85684112855 100644
--- a/code/parse/sexp.cpp
+++ b/code/parse/sexp.cpp
@@ -1433,10 +1433,15 @@ int alloc_sexp(const char *text, int type, int subtype, int first, int rest)
 	int sexp_const = get_operator_const(text);
 
 	if ((sexp_const == OP_TRUE) && (type == SEXP_ATOM) && (subtype == SEXP_ATOM_OPERATOR))
+	{
+		Assertion(Locked_sexp_true >= 0 && Locked_sexp_false >= 0, "SEXPs are not yet initialized!");
 		return Locked_sexp_true;
-
+	}
 	else if ((sexp_const == OP_FALSE) && (type == SEXP_ATOM) && (subtype == SEXP_ATOM_OPERATOR))
+	{
+		Assertion(Locked_sexp_true >= 0 && Locked_sexp_false >= 0, "SEXPs are not yet initialized!");
 		return Locked_sexp_false;
+	}
 
 	node = find_free_sexp();
 
@@ -1535,6 +1540,8 @@ void sexp_mark_persistent(int n)
 		return;
 	}
 
+	Assertion(Locked_sexp_true >= 0 && Locked_sexp_false >= 0, "SEXPs are not yet initialized!");
+
 	// total hack because of the true/false locked sexps -- we should make those persistent as well
 	if ( (n == Locked_sexp_true) || (n == Locked_sexp_false) ){
 		return;
@@ -1557,6 +1564,8 @@ void sexp_unmark_persistent(int n)
 		return;
 	}
 
+	Assertion(Locked_sexp_true >= 0 && Locked_sexp_false >= 0, "SEXPs are not yet initialized!");
+
 	// see sexp_mark_persistent
 	if ( (n == Locked_sexp_true) || (n == Locked_sexp_false) ){
 		return;
@@ -1579,6 +1588,8 @@ int free_one_sexp(int num)
 	Assert(Sexp_nodes[num].type != SEXP_NOT_USED);  // make sure it is actually used
 	Assert(!(Sexp_nodes[num].type & SEXP_FLAG_PERSISTENT));
 
+	Assertion(Locked_sexp_true >= 0 && Locked_sexp_false >= 0, "SEXPs are not yet initialized!");
+
 	// never free these nodes
 	if ((num == Locked_sexp_true) || (num == Locked_sexp_false))
 		return 0;
@@ -1603,6 +1614,8 @@ int free_sexp(int num, int calling_node)
 	Assert(Sexp_nodes[num].type != SEXP_NOT_USED);  // make sure it is actually used
 	Assert(!(Sexp_nodes[num].type & SEXP_FLAG_PERSISTENT));
 
+	Assertion(Locked_sexp_true >= 0 && Locked_sexp_false >= 0, "SEXPs are not yet initialized!");
+
 	// never free these nodes
 	if ((num == -1) || (num == Locked_sexp_true) || (num == Locked_sexp_false))
 		return 0;
@@ -4775,6 +4788,7 @@ int get_sexp()
 		} else {
 			const char *message = nullptr;
 			Assert(last != -1);
+			Assertion(Locked_sexp_true >= 0 && Locked_sexp_false >= 0, "SEXPs are not yet initialized!");
 
 			// Locked_sexp_true and Locked_sexp_false are only meant to represent operator
 			// nodes with no arguments, i.e. (true) and (false). If they appear as "bare"
@@ -11077,6 +11091,8 @@ int eval_perform_actions(int n, int op_num)
 	bool retval_first;
 	Assert( n >= 0 );
 
+	Assertion(Locked_sexp_true >= 0 && Locked_sexp_false >= 0, "SEXPs are not yet initialized!");
+
 	if (op_num == OP_PERFORM_ACTIONS_BOOL_FIRST)
 	{
 		return_cond = CAR(n);
@@ -12322,6 +12338,8 @@ int sexp_num_valid_arguments( int n )
 	if (arg_handler < 0)
 		return 0;
 
+	Assertion(Locked_sexp_true >= 0 && Locked_sexp_false >= 0, "SEXPs are not yet initialized!");
+
 	// the for-* sexps require special handling: they don't list their arguments explicitly but rather generate them on-the-fly
 	auto op_const = get_operator_const(arg_handler);
 	switch (op_const)
diff --git a/code/ship/ship.cpp b/code/ship/ship.cpp
index 0bed76538e3..d8799c4df66 100644
--- a/code/ship/ship.cpp
+++ b/code/ship/ship.cpp
@@ -7595,6 +7595,8 @@ static void ship_set(int ship_index, int objnum, int ship_type)
 	ship_weapon	*swp = &shipp->weapons;
 	polymodel *pm = model_get(sip->model_num);
 
+	Assertion(Locked_sexp_true >= 0 && Locked_sexp_false >= 0, "SEXPs are not yet initialized!");
+
 	Assert(strlen(shipp->ship_name) <= NAME_LENGTH - 1);
 	shipp->ship_info_index = ship_type;
 	shipp->objnum = objnum;
diff --git a/fred2/FictionViewerDlg.cpp b/fred2/FictionViewerDlg.cpp
index f4ec1df173d..21aecb1778b 100644
--- a/fred2/FictionViewerDlg.cpp
+++ b/fred2/FictionViewerDlg.cpp
@@ -82,6 +82,8 @@ BOOL FictionViewerDlg::OnInitDialog()
 
 void FictionViewerDlg::OnOK() 
 {
+	Assertion(Locked_sexp_true >= 0 && Locked_sexp_false >= 0, "SEXPs are not yet initialized!");
+
 	UpdateData(TRUE);
 
 	// remove whitespace
diff --git a/fred2/briefingeditordlg.cpp b/fred2/briefingeditordlg.cpp
index 1f62d1c798f..6531384c88c 100644
--- a/fred2/briefingeditordlg.cpp
+++ b/fred2/briefingeditordlg.cpp
@@ -929,6 +929,8 @@ void briefing_editor_dlg::OnInsertStage()
 
 void briefing_editor_dlg::copy_stage(int from, int to)
 {
+	Assertion(Locked_sexp_true >= 0 && Locked_sexp_false >= 0, "SEXPs are not yet initialized!");
+
 	if ((from < 0) || (from >= Briefing->num_stages)) {
 		Briefing->stages[to].text = "<Text here>";
 		strcpy_s(Briefing->stages[to].voice, "none.wav");
diff --git a/fred2/campaigntreeview.cpp b/fred2/campaigntreeview.cpp
index 25683bd7d1e..d8df18e5fc2 100644
--- a/fred2/campaigntreeview.cpp
+++ b/fred2/campaigntreeview.cpp
@@ -749,6 +749,8 @@ void campaign_tree_view::OnLButtonUp(UINT nFlags, CPoint point)
 
 int campaign_tree_view::add_link(int from, int to)
 {
+	Assertion(Locked_sexp_true >= 0 && Locked_sexp_false >= 0, "SEXPs are not yet initialized!");
+
 	if (Total_links >= MAX_CAMPAIGN_TREE_LINKS)
 		return -1;
 
@@ -892,6 +894,8 @@ DROPEFFECT campaign_tree_view::OnDragOver(COleDataObject* pDataObject, DWORD dwK
 
 BOOL campaign_tree_view::OnDrop(COleDataObject* pDataObject, DROPEFFECT dropEffect, CPoint point) 
 {
+	Assertion(Locked_sexp_true >= 0 && Locked_sexp_false >= 0, "SEXPs are not yet initialized!");
+
 	int i, level, pos;
 	cmission *cm;
 	HGLOBAL hGlobal;
@@ -999,6 +1003,8 @@ BOOL campaign_tree_view::OnDrop(COleDataObject* pDataObject, DROPEFFECT dropEffe
 
 void campaign_tree_view::drop_mission(int m, CPoint point)
 {
+	Assertion(Locked_sexp_true >= 0 && Locked_sexp_false >= 0, "SEXPs are not yet initialized!");
+
 	char name[MAX_FILENAME_LEN + 1];
 	int i, item, level, pos;
 	cmission *cm;
diff --git a/fred2/campaigntreewnd.cpp b/fred2/campaigntreewnd.cpp
index 527fcf37e50..a5896ff6930 100644
--- a/fred2/campaigntreewnd.cpp
+++ b/fred2/campaigntreewnd.cpp
@@ -367,6 +367,8 @@ void campaign_tree_wnd::OnErrorChecker()
 
 int campaign_tree_wnd::error_checker()
 {
+	Assertion(Locked_sexp_true >= 0 && Locked_sexp_false >= 0, "SEXPs are not yet initialized!");
+
 	int i, j, z;
 	int mcount[MAX_CAMPAIGN_MISSIONS], true_at[MAX_CAMPAIGN_MISSIONS];
 
diff --git a/fred2/freddoc.cpp b/fred2/freddoc.cpp
index 37f028d269b..e5764287395 100644
--- a/fred2/freddoc.cpp
+++ b/fred2/freddoc.cpp
@@ -225,6 +225,8 @@ void CFREDDoc::editor_init_mission() {
 }
 
 bool CFREDDoc::load_mission(const char *pathname, int flags) {
+	Assertion(Locked_sexp_true >= 0 && Locked_sexp_false >= 0, "SEXPs are not yet initialized!");
+
 	// make sure we're in the correct working directory!!!!!!
 	chdir(Fred_base_dir);
 
diff --git a/fred2/fredview.cpp b/fred2/fredview.cpp
index fba2463281b..2ef0d4d8a98 100644
--- a/fred2/fredview.cpp
+++ b/fred2/fredview.cpp
@@ -3138,6 +3138,8 @@ int CFREDView::global_error_check_mixed_player_wing(int w)
 
 int CFREDView::global_error_check_player_wings(int multi)
 {
+	Assertion(Locked_sexp_true >= 0 && Locked_sexp_false >= 0, "SEXPs are not yet initialized!");
+
 	int i, z, err;
 	int starting_wing_count[MAX_STARTING_WINGS];
 	int tvt_wing_count[MAX_TVT_WINGS];
diff --git a/fred2/initialstatus.cpp b/fred2/initialstatus.cpp
index fbc642d83b2..5c0f136a2ce 100644
--- a/fred2/initialstatus.cpp
+++ b/fred2/initialstatus.cpp
@@ -957,6 +957,8 @@ void initial_status::dock(object *objp, int dockpoint, object *other_objp, int o
 
 void initial_status::undock(object *objp1, object *objp2)
 {
+	Assertion(Locked_sexp_true >= 0 && Locked_sexp_false >= 0, "SEXPs are not yet initialized!");
+
 	vec3d v;
 	int ship_num, other_ship_num;
 
@@ -1005,6 +1007,8 @@ void initial_status::undock(object *objp1, object *objp2)
 
 bool set_cue_to_false(int *cue)
 {
+	Assertion(Locked_sexp_true >= 0 && Locked_sexp_false >= 0, "SEXPs are not yet initialized!");
+
 	// if the cue is not false, make it false.  Be sure to set all ship editor dialog functions
 	// to update data before and after we modify the cue.
 	if (*cue != Locked_sexp_false)
@@ -1058,6 +1062,8 @@ void reset_arrival_to_false(int shipnum, bool reset_wing)
 // group with a non-false arrival cue
 void initial_status_mark_dock_leader_helper(object *objp, dock_function_info *infop)
 {
+	Assertion(Locked_sexp_true >= 0 && Locked_sexp_false >= 0, "SEXPs are not yet initialized!");
+
 	ship *shipp = &Ships[objp->instance];
 	int cue_to_check;
 
diff --git a/fred2/management.cpp b/fred2/management.cpp
index e1fbdc50455..7919242d40b 100644
--- a/fred2/management.cpp
+++ b/fred2/management.cpp
@@ -810,6 +810,8 @@ int create_object(vec3d *pos, int waypoint_instance, bool prop)
 
 int create_player(vec3d *pos, matrix *orient, int type)
 {
+	Assertion(Locked_sexp_true >= 0 && Locked_sexp_false >= 0, "SEXPs are not yet initialized!");
+
 	int obj;
 
 	if (type == -1){
diff --git a/fred2/reinforcementeditordlg.cpp b/fred2/reinforcementeditordlg.cpp
index af06e0fa30e..d84443f922b 100644
--- a/fred2/reinforcementeditordlg.cpp
+++ b/fred2/reinforcementeditordlg.cpp
@@ -180,6 +180,8 @@ int reinforcement_editor_dlg::query_modified()
 
 void reinforcement_editor_dlg::OnOK()
 {
+	Assertion(Locked_sexp_true >= 0 && Locked_sexp_false >= 0, "SEXPs are not yet initialized!");
+
 	int i, j;
 
 	save_data();
diff --git a/fred2/wing.cpp b/fred2/wing.cpp
index 9c0d0a74606..2cd33f6f215 100644
--- a/fred2/wing.cpp
+++ b/fred2/wing.cpp
@@ -83,6 +83,8 @@ int check_wing_dependencies(int wing_num) {
 }
 
 int create_wing() {
+	Assertion(Locked_sexp_true >= 0 && Locked_sexp_false >= 0, "SEXPs are not yet initialized!");
+
 	char msg[1024];
 	int i, ship, wing = -1, waypoints = 0, count = 0, illegal_ships = 0;
 	int leader, leader_team;
diff --git a/fred2/wing_editor.cpp b/fred2/wing_editor.cpp
index a878a0d180f..455c949d392 100644
--- a/fred2/wing_editor.cpp
+++ b/fred2/wing_editor.cpp
@@ -274,6 +274,8 @@ void wing_editor::OnClose()
 // initialize everything that update_data_safe() saves.
 void wing_editor::initialize_data_safe(int full_update)
 {
+	Assertion(Locked_sexp_true >= 0 && Locked_sexp_false >= 0, "SEXPs are not yet initialized!");
+
 	int i, enable = TRUE, player_wing = 0, player_enabled = 1;
 	CComboBox *arrival_box, *departure_box;
 
@@ -575,6 +577,8 @@ void wing_editor::initialize_data(int full_update)
 // Once the error no longer occurs, bypass mode is cleared and data is updated.
 int wing_editor::update_data(int redraw)
 {
+	Assertion(Locked_sexp_true >= 0 && Locked_sexp_false >= 0, "SEXPs are not yet initialized!");
+
 	char *str, old_name[255], buf[512];
 	int i, z;
 	object *ptr;