diff --git a/compiler/rustc_const_eval/src/interpret/cast.rs b/compiler/rustc_const_eval/src/interpret/cast.rs
index 2be5ed896ec80..c14152a916a29 100644
--- a/compiler/rustc_const_eval/src/interpret/cast.rs
+++ b/compiler/rustc_const_eval/src/interpret/cast.rs
@@ -67,12 +67,12 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             }
 
             Pointer(PointerCast::ReifyFnPointer) => {
+                // All reifications must be monomorphic, bail out otherwise.
+                ensure_monomorphic_enough(*self.tcx, src.layout.ty)?;
+
                 // The src operand does not matter, just its type
                 match *src.layout.ty.kind() {
                     ty::FnDef(def_id, substs) => {
-                        // All reifications must be monomorphic, bail out otherwise.
-                        ensure_monomorphic_enough(*self.tcx, src.layout.ty)?;
-
                         let instance = ty::Instance::resolve_for_fn_ptr(
                             *self.tcx,
                             self.param_env,
@@ -100,12 +100,12 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             }
 
             Pointer(PointerCast::ClosureFnPointer(_)) => {
+                // All reifications must be monomorphic, bail out otherwise.
+                ensure_monomorphic_enough(*self.tcx, src.layout.ty)?;
+
                 // The src operand does not matter, just its type
                 match *src.layout.ty.kind() {
                     ty::Closure(def_id, substs) => {
-                        // All reifications must be monomorphic, bail out otherwise.
-                        ensure_monomorphic_enough(*self.tcx, src.layout.ty)?;
-
                         let instance = ty::Instance::resolve_closure(
                             *self.tcx,
                             def_id,
@@ -359,8 +359,11 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 let val = Immediate::new_dyn_trait(ptr, vtable, &*self.tcx);
                 self.write_immediate(val, dest)
             }
-
             _ => {
+                // Do not ICE if we are not monomorphic enough.
+                ensure_monomorphic_enough(*self.tcx, src.layout.ty)?;
+                ensure_monomorphic_enough(*self.tcx, cast_ty)?;
+
                 span_bug!(
                     self.cur_span(),
                     "invalid pointer unsizing {:?} -> {:?}",
@@ -404,12 +407,18 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 }
                 Ok(())
             }
-            _ => span_bug!(
-                self.cur_span(),
-                "unsize_into: invalid conversion: {:?} -> {:?}",
-                src.layout,
-                dest.layout
-            ),
+            _ => {
+                // Do not ICE if we are not monomorphic enough.
+                ensure_monomorphic_enough(*self.tcx, src.layout.ty)?;
+                ensure_monomorphic_enough(*self.tcx, cast_ty.ty)?;
+
+                span_bug!(
+                    self.cur_span(),
+                    "unsize_into: invalid conversion: {:?} -> {:?}",
+                    src.layout,
+                    dest.layout
+                )
+            }
         }
     }
 }
diff --git a/tests/ui/mir/unsize-trait.rs b/tests/ui/mir/unsize-trait.rs
new file mode 100644
index 0000000000000..45b5308c0938f
--- /dev/null
+++ b/tests/ui/mir/unsize-trait.rs
@@ -0,0 +1,15 @@
+// Check that the interpreter does not ICE when trying to unsize `B` to `[u8]`.
+// This is a `build` test to ensure that const-prop-lint runs.
+// build-pass
+
+#![feature(unsize)]
+
+fn foo<B>(buffer: &mut [B; 2])
+    where B: std::marker::Unsize<[u8]>,
+{
+    let buffer: &[u8] = &buffer[0];
+}
+
+fn main() {
+    foo(&mut [[0], [5]]);
+}