diff --git a/src/saveload/tracerestrict_sl.cpp b/src/saveload/tracerestrict_sl.cpp
index 93d43cf3ea..8e983a78b3 100644
--- a/src/saveload/tracerestrict_sl.cpp
+++ b/src/saveload/tracerestrict_sl.cpp
@@ -11,9 +11,10 @@
 
 #include "../stdafx.h"
 #include "../tracerestrict.h"
+#include "../strings_func.h"
+#include "../string_func.h"
 #include "saveload.h"
 #include <vector>
-#include "saveload.h"
 
 static const SaveLoad _trace_restrict_mapping_desc[] = {
   SLE_VAR(TraceRestrictMappingItem, program_id, SLE_UINT32),
@@ -66,7 +67,20 @@ static void Load_TRRP()
 		SlObject(&stub, _trace_restrict_program_stub_desc);
 		prog->items.resize(stub.length);
 		SlArray(&(prog->items[0]), stub.length, SLE_UINT32);
-		assert(prog->Validate().Succeeded());
+		CommandCost validation_result = prog->Validate();
+		if (validation_result.Failed()) {
+			char str[4096];
+			char *strend = str + seprintf(str, lastof(str), "Trace restrict program %d: %s\nProgram dump:",
+					index, GetStringPtr(validation_result.GetErrorMessage()));
+			for (unsigned int i = 0; i < prog->items.size(); i++) {
+				if (i % 3) {
+					strend += seprintf(strend, lastof(str), " %08X", prog->items[i]);
+				} else {
+					strend += seprintf(strend, lastof(str), "\n%4u: %08X", i, prog->items[i]);
+				}
+			}
+			SlErrorCorrupt(str);
+		}
 	}
 }
 
diff --git a/src/tracerestrict.cpp b/src/tracerestrict.cpp
index 85c0fd0631..c69da07ce8 100644
--- a/src/tracerestrict.cpp
+++ b/src/tracerestrict.cpp
@@ -414,7 +414,7 @@ void TraceRestrictProgram::DecrementRefCount() {
  * Validate a instruction list
  * Returns successful result if program seems OK
  * This only validates that conditional nesting is correct,
- * and that all non-conditionals have a known type, at present
+ * and that all instructions have a known type, at present
  */
 CommandCost TraceRestrictProgram::Validate(const std::vector<TraceRestrictItem> &items, TraceRestrictProgramActionsUsedFlags &actions_used_flags) {
 	// static to avoid needing to re-alloc/resize on each execution
@@ -464,6 +464,23 @@ CommandCost TraceRestrictProgram::Validate(const std::vector<TraceRestrictItem>
 				}
 				HandleCondition(condstack, condflags, true);
 			}
+
+			switch (GetTraceRestrictType(item)) {
+				case TRIT_COND_ENDIF:
+				case TRIT_COND_UNDEFINED:
+				case TRIT_COND_TRAIN_LENGTH:
+				case TRIT_COND_MAX_SPEED:
+				case TRIT_COND_CURRENT_ORDER:
+				case TRIT_COND_NEXT_ORDER:
+				case TRIT_COND_LAST_STATION:
+				case TRIT_COND_CARGO:
+				case TRIT_COND_ENTRY_DIRECTION:
+				case TRIT_COND_PBS_ENTRY_SIGNAL:
+					break;
+
+				default:
+					return_cmd_error(STR_TRACE_RESTRICT_ERROR_VALIDATE_UNKNOWN_INSTRUCTION);
+			}
 		} else {
 			switch (GetTraceRestrictType(item)) {
 				case TRIT_PF_DENY: