Now I have the context. The `else` block is on lines 741-742. Let me create the correct patch:

--- a/sympy/utilities/lambdify.py
+++ b/sympy/utilities/lambdify.py
@@ -739,7 +739,14 @@
             argstrs.append(self._argrepr(dummy))
             expr = self._subexpr(expr, {arg: dummy})
         else:
-            argstrs.append(str(arg))
+            argrep = str(arg)
+
+            if dummify or not self._is_safe_ident(argrep):
+                dummy = Dummy()
+                argstrs.append(self._argrepr(dummy))
+                expr = self._subexpr(expr, {arg: dummy})
+            else:
+                argstrs.append(argrep)
 
         return argstrs, expr

--- a/sympy/utilities/lambdify.py
+++ b/sympy/utilities/lambdify.py
@@ -738,8 +738,15 @@ class _EvaluatorPrinter(object):
             dummy = Dummy()
             argstrs.append(self._argrepr(dummy))
             expr = self._subexpr(expr, {arg: dummy})
         else:
-            argstrs.append(str(arg))
+            argrep = str(arg)
+
+            if dummify or not self._is_safe_ident(argrep):
+                dummy = Dummy()
+                argstrs.append(self._argrepr(dummy))
+                expr = self._subexpr(expr, {arg: dummy})
+            else:
+                argstrs.append(argrep)
 
         return argstrs, expr