boll.diff

diff --git a/src/core/coerce.c b/src/core/coerce.c
index c74fd67..b3e121d 100644
--- a/src/core/coerce.c
+++ b/src/core/coerce.c
@@ -71,9 +71,16 @@ void MVM_coerce_istrue(MVMThreadContext *tc, MVMObject *obj, MVMRegister *res_re
             case MVM_BOOL_MODE_UNBOX_NUM:
                 result = !IS_CONCRETE(obj) || REPR(obj)->box_funcs.get_num(tc, STABLE(obj), obj, OBJECT_BODY(obj)) == 0.0 ? 0 : 1;
                 break;
-            case MVM_BOOL_MODE_UNBOX_STR_NOT_EMPTY:
-                result = !IS_CONCRETE(obj) || MVM_string_graphs(tc, REPR(obj)->box_funcs.get_str(tc, STABLE(obj), obj, OBJECT_BODY(obj))) == 0 ? 0 : 1;
+            case MVM_BOOL_MODE_UNBOX_STR_NOT_EMPTY: {
+                MVMString *str;
+                if (!IS_CONCRETE(obj)) {
+                    result = 0;
+                    break;
+                }
+                str = REPR(obj)->box_funcs.get_str(tc, STABLE(obj), obj, OBJECT_BODY(obj));
+                result = MVM_coerce_istrue_s(tc, str);
                 break;
+            }
             case MVM_BOOL_MODE_UNBOX_STR_NOT_EMPTY_OR_ZERO: {
                 MVMString *str;
                 if (!IS_CONCRETE(obj)) {
@@ -81,7 +88,10 @@ void MVM_coerce_istrue(MVMThreadContext *tc, MVMObject *obj, MVMRegister *res_re
                     break;
                 }
                 str = REPR(obj)->box_funcs.get_str(tc, STABLE(obj), obj, OBJECT_BODY(obj));
-                result = MVM_coerce_istrue_s(tc, str);
+                result = str == NULL ||
+                        !IS_CONCRETE(str) ||
+                        (MVM_string_graphs(tc, str) == 1 && MVM_string_get_grapheme_at_nocheck(tc, str, 0) == 48)
+                        ? 0 : 1;
                 break;
             }
             case MVM_BOOL_MODE_NOT_TYPE_OBJECT: