1 files changed, 2 insertions, 0 deletions
diff --git a/stack.c b/stack.c
index 1edb5c5..f0624c8 100644
--- a/stack.c
+++ b/stack.c
@@ -18,11 +18,13 @@ int ps_push(PositionStack *ps, Position pos) {
 }
 
 Position ps_pop(PositionStack *ps) {
+    if (ps->top == 0) error("Stack underflow\n");
     ps->top -= 1;
     return ps->arr[ps->top];
 }
 
 Position ps_peek(PositionStack ps) {
+    if (ps.top == 0) error("Stack underflow\n");
     return ps.arr[ps.top - 1];
 }