diff --git a/modules/juce_core/text/juce_String.cpp b/modules/juce_core/text/juce_String.cpp
index 02cd66e299..d48d56d522 100644
--- a/modules/juce_core/text/juce_String.cpp
+++ b/modules/juce_core/text/juce_String.cpp
@@ -1387,6 +1387,10 @@ String String::replaceCharacter (const juce_wchar charToReplace, const juce_wcha
 
 String String::replaceCharacters (StringRef charactersToReplace, StringRef charactersToInsertInstead) const
 {
+    // Each character in the first string must have a matching one in the
+    // second, so the two strings must be the same length.
+    jassert (charactersToReplace.length() == charactersToInsertInstead.length());
+
     StringCreationHelper builder (text);
 
     for (;;)