diff --git a/src/errors.h b/src/errors.h
index 351ced9..0099431 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -258,8 +258,9 @@
 EXTERN char e_using_loadkeymap_not_in_sourced_file[]
 	INIT(= N_("E105: Using :loadkeymap not in a sourced file"));
 #endif
-// E106 unused
 #ifdef FEAT_EVAL
+EXTERN char e_unsupported_diff_output_format_str[]
+	INIT(= N_("E106: Unsupported diff output format: %s"));
 EXTERN char e_missing_parenthesis_str[]
 	INIT(= N_("E107: Missing parentheses: %s"));
 EXTERN char e_no_such_variable_str[]