diff --git a/src/regexp.c b/src/regexp.c
index 9d2d441..e372dd4 100644
--- a/src/regexp.c
+++ b/src/regexp.c
@@ -2069,6 +2069,9 @@
 		}
 		clear_tv(&rettv);
 	    }
+	    else if (substitute_instr != NULL)
+		// Execute instructions from ISN_SUBSTITUTE.
+		eval_result = exe_substitute_instr();
 	    else
 		eval_result = eval_to_string(source + 2, TRUE);