diff --git a/python/BioSimSpace/Sandpit/Exscientia/Protocol/_config.py b/python/BioSimSpace/Sandpit/Exscientia/Protocol/_config.py
index 300c6af8b..4496107d7 100644
--- a/python/BioSimSpace/Sandpit/Exscientia/Protocol/_config.py
+++ b/python/BioSimSpace/Sandpit/Exscientia/Protocol/_config.py
@@ -293,7 +293,7 @@ def generateAmberConfig(self, extra_options=None, extra_lines=None):
                                     ]
                                 restraint_mask = "@" + ",".join(restraint_atom_names)
                             elif restraint == "heavy":
-                                restraint_mask = "!:WAT & !@H"
+                                restraint_mask = "!:WAT & !@H="
                             elif restraint == "all":
                                 restraint_mask = "!:WAT"
 
diff --git a/python/BioSimSpace/_Config/_amber.py b/python/BioSimSpace/_Config/_amber.py
index 4f23b72c7..ddd51ed41 100644
--- a/python/BioSimSpace/_Config/_amber.py
+++ b/python/BioSimSpace/_Config/_amber.py
@@ -224,7 +224,7 @@ def createConfig(
                                     ]
                                 restraint_mask = "@" + ",".join(restraint_atom_names)
                             elif restraint == "heavy":
-                                restraint_mask = "!:WAT & !@H"
+                                restraint_mask = "!:WAT & !@H="
                             elif restraint == "all":
                                 restraint_mask = "!:WAT"