diff --git a/integrators/kw1ab.py b/integrators/kw1ab.py
index 35b144b0d1d06859d3f62cf1a6fee398915f8713..75176ce87f766c6f906c257e9073fe9def300caa 100644
--- a/integrators/kw1ab.py
+++ b/integrators/kw1ab.py
@@ -176,7 +176,10 @@ class KW1AB(kw1.KW1, _forms.lf_old.LinearForms_Old):
 
 
   def _AssembleFixPartsRHS(self):
-    self._AssemblePartsRHS()
+    if self._nowImplicit:
+      kw1.KW1._AssembleFixPartsRHS(self)
+    else:
+      self._AssemblePartsRHS()
 
 
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