diff --git a/src/tools/moc/generator.cpp b/src/tools/moc/generator.cpp
index 097d002142..625e1e6733 100644
--- a/src/tools/moc/generator.cpp
+++ b/src/tools/moc/generator.cpp
@@ -122,7 +122,7 @@ void Generator::strreg(const QByteArray &s)
 
 int Generator::stridx(const QByteArray &s)
 {
-    int i = strings.indexOf(s);
+    int i = int(strings.indexOf(s));
     Q_ASSERT_X(i != -1, Q_FUNC_INFO, "We forgot to register some strings");
     return i;
 }
@@ -841,7 +841,7 @@ void Generator::generateProperties()
         int notifyId = p.notifyId;
         if (p.notifyId < -1) {
             // signal is in parent class
-            const int indexInStrings = strings.indexOf(p.notify);
+            const int indexInStrings = int(strings.indexOf(p.notify));
             notifyId = indexInStrings | IsUnresolvedSignal;
         }
         fprintf(out, ", 0x%.8x, uint(%d), %d,\n", flags, notifyId, p.revision);
diff --git a/src/tools/moc/generator.h b/src/tools/moc/generator.h
index fa651f04a0..ac29557769 100644
--- a/src/tools/moc/generator.h
+++ b/src/tools/moc/generator.h
@@ -20,6 +20,8 @@ public:
               const QHash<QByteArray, QByteArray> &knownGadgets, FILE *outfile = nullptr,
               bool requireCompleteTypes = false);
     void generateCode();
+    qsizetype registeredStringsCount() { return strings.size(); };
+
 private:
     bool registerableMetaType(const QByteArray &propertyType);
     void registerClassInfoStrings();
diff --git a/src/tools/moc/moc.cpp b/src/tools/moc/moc.cpp
index 4a9e3463ac..d04f2d079e 100644
--- a/src/tools/moc/moc.cpp
+++ b/src/tools/moc/moc.cpp
@@ -1138,6 +1138,12 @@ void Moc::generate(FILE *out, FILE *jsonOutput)
     for (int i = 0; i < classList.size(); ++i) {
         Generator generator(&classList[i], metaTypes, knownQObjectClasses, knownGadgets, out, requireCompleteTypes);
         generator.generateCode();
+
+        // generator.generateCode() should have already registered all strings
+        if (Q_UNLIKELY(generator.registeredStringsCount() >= std::numeric_limits<int>::max())) {
+            error("internal limit exceeded: number of parsed strings is too big.");
+            exit(EXIT_FAILURE);
+        }
     }
     fputs("", out);