diff --git a/src/librustc_mir/interpret/validity.rs b/src/librustc_mir/interpret/validity.rs
index 4f90f83b735d1..7eb05ba2d2c54 100644
--- a/src/librustc_mir/interpret/validity.rs
+++ b/src/librustc_mir/interpret/validity.rs
@@ -15,7 +15,7 @@ use rustc_middle::mir::interpret::{InterpError, InterpErrorInfo};
 use rustc_middle::ty;
 use rustc_middle::ty::layout::TyAndLayout;
 use rustc_span::symbol::{sym, Symbol};
-use rustc_target::abi::{Abi, LayoutOf, Scalar, VariantIdx, Variants};
+use rustc_target::abi::{Abi, LayoutOf, Scalar, Size, VariantIdx, Variants};
 
 use std::hash::Hash;
 
@@ -744,10 +744,11 @@ impl<'rt, 'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> ValueVisitor<'mir, 'tcx, M>
         match op.layout.ty.kind {
             ty::Str => {
                 let mplace = op.assert_mem_place(self.ecx); // strings are never immediate
+                let len = mplace.len(self.ecx)?;
                 try_validation!(
-                    self.ecx.read_str(mplace),
+                    self.ecx.memory.read_bytes(mplace.ptr, Size::from_bytes(len)),
                     self.path,
-                    err_ub!(InvalidStr(..)) => { "uninitialized or non-UTF-8 data in str" },
+                    err_ub!(InvalidUninitBytes(..)) => { "uninitialized data in `str`" },
                 );
             }
             ty::Array(tys, ..) | ty::Slice(tys)
diff --git a/src/test/ui/consts/const-eval/ub-wide-ptr.rs b/src/test/ui/consts/const-eval/ub-wide-ptr.rs
index 29ac32fcf2204..f69f6a1109f76 100644
--- a/src/test/ui/consts/const-eval/ub-wide-ptr.rs
+++ b/src/test/ui/consts/const-eval/ub-wide-ptr.rs
@@ -42,11 +42,11 @@ const MY_STR_LENGTH_PTR: &MyStr = unsafe { mem::transmute((&42u8, &3)) };
 const MY_STR_MUCH_TOO_LONG: &MyStr = unsafe { mem::transmute((&42u8, usize::MAX)) };
 //~^ ERROR it is undefined behavior to use this value
 
-// invalid UTF-8
-const STR_NO_UTF8: &str = unsafe { mem::transmute::<&[u8], _>(&[0xFF]) };
+// uninitialized byte
+const STR_NO_INIT: &str = unsafe { mem::transmute::<&[_], _>(&[MaybeUninit::<u8> { uninit: () }]) };
 //~^ ERROR it is undefined behavior to use this value
-// invalid UTF-8 in user-defined str-like
-const MYSTR_NO_UTF8: &MyStr = unsafe { mem::transmute::<&[u8], _>(&[0xFF]) };
+// uninitialized byte in user-defined str-like
+const MYSTR_NO_INIT: &MyStr = unsafe { mem::transmute::<&[_], _>(&[MaybeUninit::<u8> { uninit: () }]) };
 //~^ ERROR it is undefined behavior to use this value
 
 // # slice
diff --git a/src/test/ui/consts/const-eval/ub-wide-ptr.stderr b/src/test/ui/consts/const-eval/ub-wide-ptr.stderr
index 063ea81036b69..47d29ffc9b3c1 100644
--- a/src/test/ui/consts/const-eval/ub-wide-ptr.stderr
+++ b/src/test/ui/consts/const-eval/ub-wide-ptr.stderr
@@ -41,16 +41,16 @@ LL | const MY_STR_MUCH_TOO_LONG: &MyStr = unsafe { mem::transmute((&42u8, usize:
 error[E0080]: it is undefined behavior to use this value
   --> $DIR/ub-wide-ptr.rs:46:1
    |
-LL | const STR_NO_UTF8: &str = unsafe { mem::transmute::<&[u8], _>(&[0xFF]) };
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered uninitialized or non-UTF-8 data in str at .<deref>
+LL | const STR_NO_INIT: &str = unsafe { mem::transmute::<&[_], _>(&[MaybeUninit::<u8> { uninit: () }]) };
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered uninitialized data in `str` at .<deref>
    |
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
 
 error[E0080]: it is undefined behavior to use this value
   --> $DIR/ub-wide-ptr.rs:49:1
    |
-LL | const MYSTR_NO_UTF8: &MyStr = unsafe { mem::transmute::<&[u8], _>(&[0xFF]) };
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered uninitialized or non-UTF-8 data in str at .<deref>.0
+LL | const MYSTR_NO_INIT: &MyStr = unsafe { mem::transmute::<&[_], _>(&[MaybeUninit::<u8> { uninit: () }]) };
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered uninitialized data in `str` at .<deref>.0
    |
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.