--- a/layout/base/nsBidi.cpp
+++ b/layout/base/nsBidi.cpp
@@ -664,17 +664,17 @@ void nsBidi::ResolveExplicitLevels(nsBid
 
     /* (X1) level is set for all codes, embeddingLevel keeps track of the push/pop operations */
     /* both variables may carry the NSBIDI_LEVEL_OVERRIDE flag to indicate the override status */
     nsBidiLevel embeddingLevel = level, newLevel;
     nsBidiLevel previousLevel = level;     /* previous level for regular (not CC) characters */
 
     uint16_t stack[NSBIDI_MAX_EXPLICIT_LEVEL + 2];   /* we never push anything >=NSBIDI_MAX_EXPLICIT_LEVEL
                                                         but we need one more entry as base */
-    uint32_t stackLast = 0;
+    int32_t stackLast = 0;
     int32_t overflowIsolateCount = 0;
     int32_t overflowEmbeddingCount = 0;
     int32_t validIsolateCount = 0;
 
     stack[0] = level;
 
     /* recalculate the flags */
     flags=0;
@@ -768,18 +768,24 @@ void nsBidi::ResolveExplicitLevels(nsBid
             dirProps[i] |= IGNORE_CC;
             overflowIsolateCount--;
           } else if (validIsolateCount) {
             overflowEmbeddingCount = 0;
             while (stack[stackLast] < ISOLATE) {
               /* pop embedding entries        */
               /* until the last isolate entry */
               stackLast--;
+
+              // Since validIsolateCount is true, there must be an isolate entry
+              // on the stack, so the stack is guaranteed to not be empty.
+              // Still, to eliminate a warning from coverity, we use an assertion.
+              MOZ_ASSERT(stackLast > 0);
             }
             stackLast--;  /* pop also the last isolate entry */
+            MOZ_ASSERT(stackLast >= 0);  // For coverity
             validIsolateCount--;
           } else {
             dirProps[i] |= IGNORE_CC;
           }
           embeddingLevel = stack[stackLast] & ~ISOLATE;
           previousLevel = level = embeddingLevel;
           flags |= DIRPROP_FLAG(O_N) | DIRPROP_FLAG(BN) | DIRPROP_FLAG_LR(embeddingLevel);
           break;