diff --git a/compiler/rustc_middle/src/ty/generics.rs b/compiler/rustc_middle/src/ty/generics.rs
index 85d9db7ee7473..5610067cd871e 100644
--- a/compiler/rustc_middle/src/ty/generics.rs
+++ b/compiler/rustc_middle/src/ty/generics.rs
@@ -219,11 +219,19 @@ impl<'tcx> Generics {
 
     /// Returns the `GenericParamDef` with the given index.
     pub fn param_at(&'tcx self, param_index: usize, tcx: TyCtxt<'tcx>) -> &'tcx GenericParamDef {
+        self.try_param_at(param_index, tcx).expect("bug: invalid parameter index")
+    }
+
+    pub fn try_param_at(
+        &'tcx self,
+        param_index: usize,
+        tcx: TyCtxt<'tcx>,
+    ) -> Option<&'tcx GenericParamDef> {
         if let Some(index) = param_index.checked_sub(self.parent_count) {
-            &self.own_params[index]
+            self.own_params.get(index)
         } else {
             tcx.generics_of(self.parent.expect("parent_count > 0 but no parent?"))
-                .param_at(param_index, tcx)
+                .try_param_at(param_index, tcx)
         }
     }
 
@@ -260,6 +268,16 @@ impl<'tcx> Generics {
         }
     }
 
+    pub fn try_type_param(
+        &'tcx self,
+        param: ParamTy,
+        tcx: TyCtxt<'tcx>,
+    ) -> Option<&'tcx GenericParamDef> {
+        self.try_param_at(param.index as usize, tcx).and_then(|param| {
+            if matches!(param.kind, GenericParamDefKind::Type { .. }) { Some(param) } else { None }
+        })
+    }
+
     /// Returns the `GenericParamDef` associated with this `ParamConst`.
     pub fn const_param(&'tcx self, param: ParamConst, tcx: TyCtxt<'tcx>) -> &'tcx GenericParamDef {
         let param = self.param_at(param.index as usize, tcx);
diff --git a/compiler/rustc_trait_selection/src/error_reporting/infer/note_and_explain.rs b/compiler/rustc_trait_selection/src/error_reporting/infer/note_and_explain.rs
index 514615735a507..80c55989c54bc 100644
--- a/compiler/rustc_trait_selection/src/error_reporting/infer/note_and_explain.rs
+++ b/compiler/rustc_trait_selection/src/error_reporting/infer/note_and_explain.rs
@@ -241,14 +241,17 @@ impl<T> Trait<T> for X {
                     }
                     (ty::Param(p), _) | (_, ty::Param(p)) => {
                         let generics = tcx.generics_of(body_owner_def_id);
-                        let p_span = tcx.def_span(generics.type_param(p, tcx).def_id);
-                        let expected = match (values.expected.kind(), values.found.kind()) {
-                            (ty::Param(_), _) => "expected ",
-                            (_, ty::Param(_)) => "found ",
-                            _ => "",
-                        };
-                        if !sp.contains(p_span) {
-                            diag.span_label(p_span, format!("{expected}this type parameter"));
+                        let type_param = generics.try_type_param(p, tcx);
+                        if let Some(type_param) = type_param {
+                            let p_span = tcx.def_span(type_param.def_id);
+                            let expected = match (values.expected.kind(), values.found.kind()) {
+                                (ty::Param(_), _) => "expected ",
+                                (_, ty::Param(_)) => "found ",
+                                _ => "",
+                            };
+                            if !sp.contains(p_span) {
+                                diag.span_label(p_span, format!("{expected}this type parameter"));
+                            }
                         }
                     }
                     (ty::Alias(ty::Projection | ty::Inherent, proj_ty), _)
diff --git a/tests/ui/generics/generic-param-issue-137865.rs b/tests/ui/generics/generic-param-issue-137865.rs
new file mode 100644
index 0000000000000..fa0a1900e330b
--- /dev/null
+++ b/tests/ui/generics/generic-param-issue-137865.rs
@@ -0,0 +1,9 @@
+//@ edition: 2021
+
+
+trait Foo {
+    type Assoc<const N: Self>; //~ ERROR: `Self` is forbidden as the type of a const generic parameter
+    fn foo() -> Self::Assoc<3>; //~ ERROR: mismatched types
+}
+
+fn main() {}
diff --git a/tests/ui/generics/generic-param-issue-137865.stderr b/tests/ui/generics/generic-param-issue-137865.stderr
new file mode 100644
index 0000000000000..c0bd8a5e557d6
--- /dev/null
+++ b/tests/ui/generics/generic-param-issue-137865.stderr
@@ -0,0 +1,20 @@
+error: `Self` is forbidden as the type of a const generic parameter
+  --> $DIR/generic-param-issue-137865.rs:5:25
+   |
+LL |     type Assoc<const N: Self>;
+   |                         ^^^^
+   |
+   = note: the only supported types are integers, `bool`, and `char`
+
+error[E0308]: mismatched types
+  --> $DIR/generic-param-issue-137865.rs:6:29
+   |
+LL |     fn foo() -> Self::Assoc<3>;
+   |                             ^ expected type parameter `Self`, found integer
+   |
+   = note: expected type parameter `Self`
+                        found type `{integer}`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0308`.