diff --git a/spec/lexical-structure.md b/spec/lexical-structure.md
index 2152bee..a7bde85 100644
--- a/spec/lexical-structure.md
+++ b/spec/lexical-structure.md
@@ -698,7 +698,7 @@ single_regular_balanced_text_character
     ;
     
 interpolation_format
-    : interpolation_format_character+
+    : ':' interpolation_format_character+
     ;
     
 interpolation_format_character