diff --git a/compile.c b/compile.c
index 41aee58f75570d..09a3ff946a4733 100644
--- a/compile.c
+++ b/compile.c
@@ -5565,7 +5565,7 @@ compile_hash(rb_iseq_t *iseq, LINK_ANCHOR *const ret, const NODE *node, int meth
     return 1;
 }
 
-VALUE
+static VALUE
 rb_node_case_when_optimizable_literal(const NODE *const node)
 {
     switch (nd_type(node)) {
diff --git a/prism/prism.c b/prism/prism.c
index 311291aff5b43a..511223aaeeb3e8 100644
--- a/prism/prism.c
+++ b/prism/prism.c
@@ -19816,6 +19816,18 @@ parse_expression_prefix(pm_parser_t *parser, pm_binding_power_t binding_power, u
                 }
             }
 
+            /* If a missing terminator left this heredoc's lex mode on the
+             * stack, it still points at our stack-local common_whitespace.
+             * Clear the pointer so that subsequent lexing cannot read from
+             * this function's dead stack frame. */
+            pm_lex_mode_t *whitespace_mode = parser->lex_modes.current;
+            do {
+                if (whitespace_mode->mode == PM_LEX_HEREDOC && whitespace_mode->as.heredoc.common_whitespace == &common_whitespace) {
+                    whitespace_mode->as.heredoc.common_whitespace = NULL;
+                }
+                whitespace_mode = whitespace_mode->prev;
+            } while (whitespace_mode != NULL);
+
             if (match1(parser, PM_TOKEN_STRING_BEGIN)) {
                 return parse_strings(parser, node, false, (uint16_t) (depth + 1));
             }