--- a/js/src/jsgc.cpp
+++ b/js/src/jsgc.cpp
@@ -3874,32 +3874,36 @@ IncrementalCollectSlice(JSRuntime *rt,
     AutoCopyFreeListToArenas copy(rt);
     AutoGCSlice slice(rt);
 
     gc::State initialState = rt->gcIncrementalState;
     SliceBudget sliceBudget(budget);
 
     int zeal = 0;
 #ifdef JS_GC_ZEAL
-    if (reason == gcreason::DEBUG_GC) {
+    if (reason == gcreason::DEBUG_GC && budget != SliceBudget::Unlimited) {
         /*
-         * Do the collection type specified by zeal mode only if the collection
-         * was triggered by RunDebugGC().
+         * Do the incremental collection type specified by zeal mode if the
+         * collection was triggered by RunDebugGC() and incremental GC has not
+         * been cancelled by ResetIncrementalGC.
          */
         zeal = rt->gcZeal();
-        JS_ASSERT_IF(zeal == ZealIncrementalMarkAllThenFinish ||
-                     zeal == ZealIncrementalRootsThenFinish,
-                     budget == SliceBudget::Unlimited);
     }
 #endif
 
     bool isIncremental = rt->gcIncrementalState != NO_INCREMENTAL ||
-                         budget != SliceBudget::Unlimited ||
-                         zeal == ZealIncrementalRootsThenFinish ||
-                         zeal == ZealIncrementalMarkAllThenFinish;
+                         budget != SliceBudget::Unlimited;
+
+    if (zeal == ZealIncrementalRootsThenFinish || zeal == ZealIncrementalMarkAllThenFinish) {
+        /*
+         * Yields between slices occurs at predetermined points in these
+         * modes. sliceBudget is not used.
+         */
+        sliceBudget.reset();
+    }
 
     if (rt->gcIncrementalState == NO_INCREMENTAL) {
         rt->gcIncrementalState = MARK_ROOTS;
         rt->gcLastMarkSlice = false;
     }
 
     switch (rt->gcIncrementalState) {
 
@@ -4499,17 +4503,18 @@ RunDebugGC(JSContext *cx)
              * completion.
              */
             if (initialState == NO_INCREMENTAL)
                 rt->gcIncrementalLimit = rt->gcZealFrequency / 2;
             else
                 rt->gcIncrementalLimit *= 2;
             budget = SliceBudget::WorkBudget(rt->gcIncrementalLimit);
         } else {
-            budget = SliceBudget::Unlimited;
+            // This triggers incremental GC but is actually ignored by IncrementalMarkSlice.
+            budget = SliceBudget::WorkBudget(1);
         }
 
         Collect(rt, true, budget, GC_NORMAL, gcreason::DEBUG_GC);
 
         /*
          * For multi-slice zeal, reset the slice size when we get to the sweep
          * phase.
          */