Fixes #5170: Improve msd_type parsing

package/AUTHORS

@@ -265,6 +265,7 @@ Chronological list of authors
   - Raúl Lois-Cuns  
   - Pranay Pelapkar
   - Shreejan Dolai
+  - Tanisha Dubey
 
 External code
 -------------

package/MDAnalysis/analysis/msd.py

@@ -382,10 +382,10 @@ def _parse_msd_type(self):
             "xyz": [0, 1, 2],
         }
 
-        self.msd_type = self.msd_type.lower()
-
         try:
-            self._dim = keys[self.msd_type]
+            self._dim = keys[self.msd_type.lower()]
+        except AttributeError:
+            raise TypeError("msd_type must be a string")
         except KeyError:
             raise ValueError(
                 "invalid msd_type: {} specified, please specify one of xyz, "

testsuite/MDAnalysisTests/analysis/test_msd.py

@@ -123,6 +123,15 @@ def test_msdtype_error(self, u, SELECTION, msdtype):
         with pytest.raises(ValueError, match=errmsg):
             m = MSD(u, SELECTION, msd_type=msdtype)
 
+    def test_msd_type_uppercase(self, u, SELECTION):
+        m = MSD(u, SELECTION, msd_type="Xz", fft=False)
+        assert m.dim_fac == 2
+        assert m._dim == [0, 2]
+
+    def test_msd_type_nonstring(self, u, SELECTION):
+        with pytest.raises(TypeError):
+            MSD(u, SELECTION, msd_type=123, fft=False)
+
     @pytest.mark.parametrize(
         "dim, dim_factor",
         [