diff --git a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
index a609fb2b172..672c9b7b088 100644
--- a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
@@ -288,7 +288,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 .substs
                 .const_at(i)
                 .try_eval_bool(self.tcx(), obligation.param_env)
-                .unwrap()
+                .unwrap_or(true)
         };
 
         let src_and_dst = predicate.map_bound(|p| rustc_transmute::Types {
diff --git a/src/test/ui/transmutability/malformed-program-gracefulness/wrong-type-assume.rs b/src/test/ui/transmutability/malformed-program-gracefulness/wrong-type-assume.rs
new file mode 100644
index 00000000000..bd36748e790
--- /dev/null
+++ b/src/test/ui/transmutability/malformed-program-gracefulness/wrong-type-assume.rs
@@ -0,0 +1,40 @@
+//! The implementation must behave well if const values of wrong types are
+//! provided.
+
+#![crate_type = "lib"]
+#![feature(transmutability)]
+#![allow(dead_code, incomplete_features, non_camel_case_types)]
+
+mod assert {
+    use std::mem::BikeshedIntrinsicFrom;
+
+    pub fn is_transmutable<
+        Src,
+        Dst,
+        Context,
+        const ASSUME_ALIGNMENT: bool,
+        const ASSUME_LIFETIMES: bool,
+        const ASSUME_VALIDITY: bool,
+        const ASSUME_VISIBILITY: bool,
+    >()
+    where
+        Dst: BikeshedIntrinsicFrom<
+            Src,
+            Context,
+            ASSUME_ALIGNMENT,
+            ASSUME_LIFETIMES,
+            ASSUME_VALIDITY,
+            ASSUME_VISIBILITY,
+        >,
+    {}
+}
+
+fn test() {
+    struct Context;
+    #[repr(C)] struct Src;
+    #[repr(C)] struct Dst;
+    assert::is_transmutable::<Src, Dst, Context, {0u8}, false, false, false>(); //~ ERROR mismatched types
+    assert::is_transmutable::<Src, Dst, Context, false, {0u8}, false, false>(); //~ ERROR mismatched types
+    assert::is_transmutable::<Src, Dst, Context, false, false, {0u8}, false>(); //~ ERROR mismatched types
+    assert::is_transmutable::<Src, Dst, Context, false, false, false, {0u8}>(); //~ ERROR mismatched types
+}
diff --git a/src/test/ui/transmutability/malformed-program-gracefulness/wrong-type-assume.stderr b/src/test/ui/transmutability/malformed-program-gracefulness/wrong-type-assume.stderr
new file mode 100644
index 00000000000..e1464e02352
--- /dev/null
+++ b/src/test/ui/transmutability/malformed-program-gracefulness/wrong-type-assume.stderr
@@ -0,0 +1,27 @@
+error[E0308]: mismatched types
+  --> $DIR/wrong-type-assume.rs:36:51
+   |
+LL |     assert::is_transmutable::<Src, Dst, Context, {0u8}, false, false, false>();
+   |                                                   ^^^ expected `bool`, found `u8`
+
+error[E0308]: mismatched types
+  --> $DIR/wrong-type-assume.rs:37:58
+   |
+LL |     assert::is_transmutable::<Src, Dst, Context, false, {0u8}, false, false>();
+   |                                                          ^^^ expected `bool`, found `u8`
+
+error[E0308]: mismatched types
+  --> $DIR/wrong-type-assume.rs:38:65
+   |
+LL |     assert::is_transmutable::<Src, Dst, Context, false, false, {0u8}, false>();
+   |                                                                 ^^^ expected `bool`, found `u8`
+
+error[E0308]: mismatched types
+  --> $DIR/wrong-type-assume.rs:39:72
+   |
+LL |     assert::is_transmutable::<Src, Dst, Context, false, false, false, {0u8}>();
+   |                                                                        ^^^ expected `bool`, found `u8`
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0308`.