diff --git a/pymatgen/analysis/gb/grain.py b/pymatgen/analysis/gb/grain.py
index 0b179a9b29d..ad99a0b36f0 100644
--- a/pymatgen/analysis/gb/grain.py
+++ b/pymatgen/analysis/gb/grain.py
@@ -414,8 +414,8 @@ def gb_from_parameters(self, rotation_axis, rotation_angle, expand_times=4, vacu
         elif lat_type == 'o':
             logger.info('Make sure this is for orthorhombic system')
             if ratio is None:
-                raise RuntimeError('CSL donot exist if all axial ratios are irrational'
-                                   'for orthorhombic system')
+                raise RuntimeError('CSL does not exist if all axial ratios are irrational '
+                                   'for an orthorhombic system')
             elif len(ratio) != 3:
                 raise RuntimeError('Orthorhombic system needs correct c2:b2:a2 ratio')
         elif lat_type == 'h':