diff --git a/src/librustc_mir/interpret/intern.rs b/src/librustc_mir/interpret/intern.rs
index c99f39977fe..5b19f879389 100644
--- a/src/librustc_mir/interpret/intern.rs
+++ b/src/librustc_mir/interpret/intern.rs
@@ -320,12 +320,19 @@ pub fn intern_const_alloc_recursive<M: CompileTimeMachine<'mir, 'tcx>>(
             // We can't call the `intern_shallow` method here, as its logic is tailored to safe
             // references and a `leftover_allocations` set (where we only have a todo-list here).
             // So we hand-roll the interning logic here again.
-            if base_intern_mode != InternMode::Static {
-                // If it's not a static, it *must* be immutable.
-                // We cannot have mutable memory inside a constant.
-                // FIXME: ideally we would assert that they already are immutable, to double-
-                // check our static checks.
-                alloc.mutability = Mutability::Not;
+            match base_intern_mode {
+                InternMode::Static => {}
+                InternMode::Const | InternMode::ConstBase => {
+                    // If it's not a static, it *must* be immutable.
+                    // We cannot have mutable memory inside a constant.
+                    // We use `delay_span_bug` here, because this can be reached in the presence
+                    // of fancy transmutes.
+                    if alloc.mutability == Mutability::Mut {
+                        // For better errors later, mark the allocation as immutable
+                        alloc.mutability = Mutability::Not;
+                        ecx.tcx.sess.delay_span_bug(ecx.tcx.span, "mutable allocation in constant");
+                    }
+                }
             }
             let alloc = tcx.intern_const_alloc(alloc);
             tcx.alloc_map.lock().set_alloc_id_memory(alloc_id, alloc);
@@ -337,6 +344,8 @@ pub fn intern_const_alloc_recursive<M: CompileTimeMachine<'mir, 'tcx>>(
         } else if ecx.memory.dead_alloc_map.contains_key(&alloc_id) {
             // dangling pointer
             throw_unsup!(ValidationFailure("encountered dangling pointer in final constant".into()))
+        } else if ecx.tcx.alloc_map.lock().get(alloc_id).is_none() {
+            span_bug!(ecx.tcx.span, "encountered unknown alloc id {:?}", alloc_id);
         }
     }
     Ok(())
diff --git a/src/test/ui/consts/miri_unleashed/mutable_const2.rs b/src/test/ui/consts/miri_unleashed/mutable_const2.rs
new file mode 100644
index 00000000000..97af1f2f993
--- /dev/null
+++ b/src/test/ui/consts/miri_unleashed/mutable_const2.rs
@@ -0,0 +1,19 @@
+// compile-flags: -Zunleash-the-miri-inside-of-you
+// failure-status: 101
+// rustc-env:RUST_BACKTRACE=0
+// normalize-stderr-test "note: rustc 1.* running on .*" -> "note: rustc VERSION running on TARGET"
+// normalize-stderr-test "note: compiler flags: .*" -> "note: compiler flags: FLAGS"
+// normalize-stderr-test "interpret/intern.rs:[0-9]*:[0-9]*" -> "interpret/intern.rs:LL:CC"
+
+#![feature(const_raw_ptr_deref)]
+#![feature(const_mut_refs)]
+#![deny(const_err)]
+
+use std::cell::UnsafeCell;
+
+// make sure we do not just intern this as mutable
+const MUTABLE_BEHIND_RAW: *mut i32 = &UnsafeCell::new(42) as *const _ as *mut _;
+//~^ WARN: skipping const checks
+//~| ERROR: mutable allocation in constant
+
+fn main() {}
diff --git a/src/test/ui/consts/miri_unleashed/mutable_const2.stderr b/src/test/ui/consts/miri_unleashed/mutable_const2.stderr
new file mode 100644
index 00000000000..3493b7c54c4
--- /dev/null
+++ b/src/test/ui/consts/miri_unleashed/mutable_const2.stderr
@@ -0,0 +1,25 @@
+warning: skipping const checks
+  --> $DIR/mutable_const2.rs:15:38
+   |
+LL | const MUTABLE_BEHIND_RAW: *mut i32 = &UnsafeCell::new(42) as *const _ as *mut _;
+   |                                      ^^^^^^^^^^^^^^^^^^^^
+
+error: internal compiler error: mutable allocation in constant
+  --> $DIR/mutable_const2.rs:15:1
+   |
+LL | const MUTABLE_BEHIND_RAW: *mut i32 = &UnsafeCell::new(42) as *const _ as *mut _;
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+thread 'rustc' panicked at 'no errors encountered even though `delay_span_bug` issued', src/librustc_errors/lib.rs:347:17
+note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.
+
+error: internal compiler error: unexpected panic
+
+note: the compiler unexpectedly panicked. this is a bug.
+
+note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports
+
+note: rustc VERSION running on TARGET
+
+note: compiler flags: FLAGS
+