diff --git a/tcl/target/stm32wlx.cfg b/tcl/target/stm32wlx.cfg
index edb3fb32b..fafe9bcba 100644
--- a/tcl/target/stm32wlx.cfg
+++ b/tcl/target/stm32wlx.cfg
@@ -151,7 +151,7 @@ proc stm32wlx_get_chipname {} {
 	if {$sep == -1} {
 		return $t
 	}
-	return [string range $t 0 [expr $sep - 1]]
+	return [string range $t 0 [expr {$sep - 1}]]
 }
 
 # like mrw, but with target selection