diff --git a/src/kits/support/String.cpp b/src/kits/support/String.cpp
index 4a1717f100..67dd1e7a0d 100644
--- a/src/kits/support/String.cpp
+++ b/src/kits/support/String.cpp
@@ -1787,10 +1787,12 @@ BString::LockBuffer(int32 maxLength)
 	if (maxLength > length)
 		length = maxLength;
 
-	if (_MakeWritable(length, true) == B_OK) {
-		_ReferenceCount() = -1;
-			// mark unshareable
-	}
+	if (_MakeWritable(length, true) != B_OK)
+		return NULL;
+
+	_ReferenceCount() = -1;
+		// mark unshareable
+
 	return fPrivateData;
 }