diff --git a/compiler/noirc_evaluator/src/errors.rs b/compiler/noirc_evaluator/src/errors.rs
index 40f4336e0b5..06259e06248 100644
--- a/compiler/noirc_evaluator/src/errors.rs
+++ b/compiler/noirc_evaluator/src/errors.rs
@@ -48,6 +48,8 @@ pub enum RuntimeError {
     BigIntModulus { call_stack: CallStack },
     #[error("Slices cannot be returned from an unconstrained runtime to a constrained runtime")]
     UnconstrainedSliceReturnToConstrained { call_stack: CallStack },
+    #[error("All `oracle` methods should be wrapped in an unconstrained fn")]
+    UnconstrainedOracleReturnToConstrained { call_stack: CallStack },
 }
 
 // We avoid showing the actual lhs and rhs since most of the time they are just 0
@@ -139,6 +141,7 @@ impl RuntimeError {
             | RuntimeError::NestedSlice { call_stack, .. }
             | RuntimeError::BigIntModulus { call_stack, .. }
             | RuntimeError::UnconstrainedSliceReturnToConstrained { call_stack } => call_stack,
+            RuntimeError::UnconstrainedOracleReturnToConstrained { call_stack } => call_stack,
         }
     }
 }
diff --git a/compiler/noirc_evaluator/src/ssa/acir_gen/mod.rs b/compiler/noirc_evaluator/src/ssa/acir_gen/mod.rs
index 5a4fa021f1f..8390f480e3a 100644
--- a/compiler/noirc_evaluator/src/ssa/acir_gen/mod.rs
+++ b/compiler/noirc_evaluator/src/ssa/acir_gen/mod.rs
@@ -612,9 +612,11 @@ impl Context {
                             self.ssa_values.insert(*result, output);
                         }
                     }
-                    Value::ForeignFunction(_) => unreachable!(
-                        "All `oracle` methods should be wrapped in an unconstrained fn"
-                    ),
+                    Value::ForeignFunction(_) => {
+                        return Err(RuntimeError::UnconstrainedOracleReturnToConstrained {
+                            call_stack: self.acir_context.get_call_stack(),
+                        })
+                    }
                     _ => unreachable!("expected calling a function but got {function_value:?}"),
                 }
             }
diff --git a/compiler/noirc_frontend/src/hir/type_check/errors.rs b/compiler/noirc_frontend/src/hir/type_check/errors.rs
index 7eacc8eb2d1..3d834128688 100644
--- a/compiler/noirc_frontend/src/hir/type_check/errors.rs
+++ b/compiler/noirc_frontend/src/hir/type_check/errors.rs
@@ -122,6 +122,10 @@ pub enum TypeCheckError {
         "Cannot pass a mutable reference from a constrained runtime to an unconstrained runtime"
     )]
     ConstrainedReferenceToUnconstrained { span: Span },
+    #[error(
+        "Cannot pass a mutable reference from a unconstrained runtime to an constrained runtime"
+    )]
+    UnconstrainedReferenceToConstrained { span: Span },
     #[error("Slices cannot be returned from an unconstrained runtime to a constrained runtime")]
     UnconstrainedSliceReturnToConstrained { span: Span },
     #[error("Only sized types may be used in the entry point to a program")]
@@ -229,6 +233,7 @@ impl From<TypeCheckError> for Diagnostic {
             | TypeCheckError::OverflowingAssignment { span, .. }
             | TypeCheckError::FieldModulo { span }
             | TypeCheckError::ConstrainedReferenceToUnconstrained { span }
+            | TypeCheckError::UnconstrainedReferenceToConstrained { span }
             | TypeCheckError::UnconstrainedSliceReturnToConstrained { span } => {
                 Diagnostic::simple_error(error.to_string(), String::new(), span)
             }
diff --git a/compiler/noirc_frontend/src/hir/type_check/expr.rs b/compiler/noirc_frontend/src/hir/type_check/expr.rs
index 7219f4d09c6..0b3dd022209 100644
--- a/compiler/noirc_frontend/src/hir/type_check/expr.rs
+++ b/compiler/noirc_frontend/src/hir/type_check/expr.rs
@@ -184,14 +184,18 @@ impl<'interner> TypeChecker<'interner> {
                 let return_type = self.bind_function_type(function, args, span);
 
                 // Check that we are not passing a slice from an unconstrained runtime to a constrained runtime
-                if is_current_func_constrained
-                    && is_unconstrained_call
-                    && return_type.contains_slice()
-                {
-                    self.errors.push(TypeCheckError::UnconstrainedSliceReturnToConstrained {
-                        span: self.interner.expr_span(expr_id),
-                    });
-                    return Type::Error;
+                if is_current_func_constrained && is_unconstrained_call {
+                    if return_type.contains_slice() {
+                        self.errors.push(TypeCheckError::UnconstrainedSliceReturnToConstrained {
+                            span: self.interner.expr_span(expr_id),
+                        });
+                        return Type::Error;
+                    } else if matches!(&return_type.follow_bindings(), Type::MutableReference(_)) {
+                        self.errors.push(TypeCheckError::UnconstrainedReferenceToConstrained {
+                            span: self.interner.expr_span(expr_id),
+                        });
+                        return Type::Error;
+                    }
                 }
 
                 return_type
diff --git a/test_programs/compile_failure/unconstrained_oracle/Nargo.toml b/test_programs/compile_failure/unconstrained_oracle/Nargo.toml
new file mode 100644
index 00000000000..1081b5ab8e2
--- /dev/null
+++ b/test_programs/compile_failure/unconstrained_oracle/Nargo.toml
@@ -0,0 +1,5 @@
+[package]
+name = "unconstrained_oracle"
+type = "bin"
+authors = [""]
+[dependencies]
\ No newline at end of file
diff --git a/test_programs/compile_failure/unconstrained_oracle/src/main.nr b/test_programs/compile_failure/unconstrained_oracle/src/main.nr
new file mode 100644
index 00000000000..abc7bf51ab8
--- /dev/null
+++ b/test_programs/compile_failure/unconstrained_oracle/src/main.nr
@@ -0,0 +1,9 @@
+#[oracle(getNoun)]
+unconstrained fn external_fn() -> Field {
+    100 / 5
+}
+
+fn main() {
+    let x = anon();
+    assert(x * 5 == 100);
+}
diff --git a/test_programs/compile_failure/unconstrained_ref/Nargo.toml b/test_programs/compile_failure/unconstrained_ref/Nargo.toml
new file mode 100644
index 00000000000..b120a3b5532
--- /dev/null
+++ b/test_programs/compile_failure/unconstrained_ref/Nargo.toml
@@ -0,0 +1,5 @@
+[package]
+name = "unconstrained_ref"
+type = "bin"
+authors = [""]
+[dependencies]
\ No newline at end of file
diff --git a/test_programs/compile_failure/unconstrained_ref/src/main.nr b/test_programs/compile_failure/unconstrained_ref/src/main.nr
new file mode 100644
index 00000000000..bb4b2090ddb
--- /dev/null
+++ b/test_programs/compile_failure/unconstrained_ref/src/main.nr
@@ -0,0 +1,8 @@
+unconstrained fn uncon_ref() -> &mut Field {
+    let lr = &mut 7;
+    lr
+}
+
+fn main() {
+    let e = uncon_ref();
+}
\ No newline at end of file