diff --git a/SignalMessaging/profiles/OWSProfileManager.m b/SignalMessaging/profiles/OWSProfileManager.m
index 2ab8adb80..8a137d106 100644
--- a/SignalMessaging/profiles/OWSProfileManager.m
+++ b/SignalMessaging/profiles/OWSProfileManager.m
@@ -530,6 +530,10 @@ const NSUInteger kOWSProfileManager_MaxAvatarDiameter = 640;
 - (nullable NSData *)groupIdForGroupKey:(NSString *)groupKey {
     NSMutableData *groupId = [NSMutableData new];
 
+    if (groupKey.length % 2 != 0) {
+        OWSFailDebug(@"Group key has unexpected length: %@ (%lu)", groupKey, (unsigned long)groupKey.length);
+        return nil;
+    }
     for (NSUInteger i = 0; i + 2 <= groupKey.length; i += 2) {
         NSString *_Nullable byteString = [groupKey substringWithRange:NSMakeRange(i, 2)];
         if (!byteString) {