diff --git a/clang-tools-extra/clang-tidy/utils/IncludeSorter.cpp b/clang-tools-extra/clang-tidy/utils/IncludeSorter.cpp
--- a/clang-tools-extra/clang-tidy/utils/IncludeSorter.cpp
+++ b/clang-tools-extra/clang-tidy/utils/IncludeSorter.cpp
@@ -45,8 +45,12 @@
 
     // Objective-C categories have a `+suffix` format, but should be grouped
     // with the file they are a category of.
+    size_t start_index = Canonical.find_last_of('/');
+    if (start_index == StringRef::npos) {
+      start_index = 0;
+    }
     return Canonical.substr(
-        0, Canonical.find_first_of('+', Canonical.find_last_of('/')));
+        0, Canonical.find_first_of('+', start_index));
   }
   return RemoveFirstSuffix(
       RemoveFirstSuffix(Str, {".cc", ".cpp", ".c", ".h", ".hpp"}),