diff --git a/js/src/builtin/TestingFunctions.cpp b/js/src/builtin/TestingFunctions.cpp
index 0c07d4fc2cba..22475d59717d 100644
--- a/js/src/builtin/TestingFunctions.cpp
+++ b/js/src/builtin/TestingFunctions.cpp
@@ -271,7 +271,7 @@ static const struct ParamPair {
 
 // Keep this in sync with above params.
 #define GC_PARAMETER_ARGS_LIST "maxBytes, maxMallocBytes, gcBytes, gcNumber, sliceTimeBudget, or markStackLimit"
- 
+
 static bool
 GCParameter(JSContext *cx, unsigned argc, Value *vp)
 {
@@ -320,6 +320,11 @@ GCParameter(JSContext *cx, unsigned argc, Value *vp)
         return false;
     }
 
+    if (param == JSGC_MARK_STACK_LIMIT && IsIncrementalGCInProgress(cx->runtime())) {
+        JS_ReportError(cx, "attempt to set markStackLimit while a GC is in progress");
+        return false;
+    }
+
     if (param == JSGC_MAX_BYTES) {
         uint32_t gcBytes = JS_GetGCParameter(cx->runtime(), JSGC_BYTES);
         if (value < gcBytes) {