diff --git a/cores/esp32/WString.cpp b/cores/esp32/WString.cpp
index 6f0a4fc68a8..990a2824a51 100644
--- a/cores/esp32/WString.cpp
+++ b/cores/esp32/WString.cpp
@@ -677,7 +677,10 @@ int String::lastIndexOf(char ch, unsigned int fromIndex) const {
     wbuffer()[fromIndex + 1] = tempchar;
     if(temp == NULL)
         return -1;
-    return temp - buffer();
+    const int rv = temp - buffer();
+    if(rv >= len())
+        return -1;
+    return rv;
 }
 
 int String::lastIndexOf(const String &s2) const {