diff --git a/eos/effectHandlerHelpers.py b/eos/effectHandlerHelpers.py
index 539328471..d27d8ae89 100644
--- a/eos/effectHandlerHelpers.py
+++ b/eos/effectHandlerHelpers.py
@@ -18,6 +18,8 @@
 # ===============================================================================
 
 from logbook import Logger
+
+from eos.exception import HandledListActionError
 from utils.deprecated import deprecated
 
 pyfalog = Logger(__name__)
@@ -125,11 +127,11 @@ class HandledModuleList(HandledList):
                     emptyPosition = currPos
 
         if emptyPosition < len(self):
-            del self[emptyPosition]
             mod.position = emptyPosition
-            HandledList.insert(self, emptyPosition, mod)
+            self.__toModule(emptyPosition, mod)
             if mod.isInvalid:
-                self.remove(mod)
+                self.__toDummy(mod.position)
+                raise HandledListActionError(mod)
             return
 
         self.appendIgnoreEmpty(mod)
@@ -139,7 +141,17 @@ class HandledModuleList(HandledList):
         HandledList.append(self, mod)
         if mod.isInvalid:
             self.remove(mod)
-            return
+            raise HandledListActionError(mod)
+
+    def replace(self, idx, mod):
+        try:
+            oldMod = self[idx]
+        except IndexError:
+            raise HandledListActionError(mod)
+        self.__toModule(idx, mod)
+        if mod.isInvalid:
+            self.__toModule(idx, oldMod)
+            raise HandledListActionError(mod)
 
     def replaceRackPosition(self, rackPosition, mod):
         listPositions = []
@@ -152,19 +164,24 @@ class HandledModuleList(HandledList):
         except IndexError:
             self.appendIgnoreEmpty(mod)
         else:
-            self.toDummy(modListPosition)
+            self.__toDummy(modListPosition)
             if not mod.isEmpty:
-                self.toModule(modListPosition, mod)
+                self.__toModule(modListPosition, mod)
                 if mod.isInvalid:
-                    self.toDummy(modListPosition)
+                    self.__toDummy(modListPosition)
+                    raise HandledListActionError(mod)
 
-    def insert(self, index, mod):
-        mod.position = index
-        i = index
+    def insert(self, idx, mod):
+        mod.position = idx
+        i = idx
         while i < len(self):
             self[i].position += 1
             i += 1
-        HandledList.insert(self, index, mod)
+        HandledList.insert(self, idx, mod)
+        if mod.isInvalid:
+            self.remove(mod)
+            raise HandledListActionError(mod)
+
 
     def remove(self, mod):
         HandledList.remove(self, mod)
@@ -174,14 +191,17 @@ class HandledModuleList(HandledList):
         for i in range(oldPos, len(self)):
             self[i].position -= 1
 
-    def toDummy(self, index):
+    def free(self, idx):
+        self.__toDummy(idx)
+
+    def __toDummy(self, index):
         mod = self[index]
         if not mod.isEmpty:
             dummy = mod.buildEmpty(mod.slot)
             dummy.position = index
             self[index] = dummy
 
-    def toModule(self, index, mod):
+    def __toModule(self, index, mod):
         mod.position = index
         self[index] = mod
 
@@ -190,7 +210,7 @@ class HandledModuleList(HandledList):
         for i in range(len(self)):
             mod = self[i]
             if mod.getModifiedItemAttr("subSystemSlot") == slot:
-                self.toDummy(i)
+                self.__toDummy(i)
                 break
 
 
diff --git a/eos/exception.py b/eos/exception.py
new file mode 100644
index 000000000..b3bb90a29
--- /dev/null
+++ b/eos/exception.py
@@ -0,0 +1,2 @@
+class HandledListActionError(Exception):
+    ...
diff --git a/gui/fitCommands/calc/fitRemoveModule.py b/gui/fitCommands/calc/fitRemoveModule.py
index b9642e831..b59b6e512 100644
--- a/gui/fitCommands/calc/fitRemoveModule.py
+++ b/gui/fitCommands/calc/fitRemoveModule.py
@@ -25,7 +25,7 @@ class FitRemoveModuleCommand(wx.Command):
             mod = fit.modules[position]
             if not mod.isEmpty:
                 self.oldModInfos[position] = ModuleInfo.fromModule(mod)
-                fit.modules.toDummy(position)
+                fit.modules.free(position)
 
         # If no modules were removed, report that command was not completed
         if not len(self.oldModInfos) > 0:
diff --git a/gui/fitCommands/calc/fitReplaceModule.py b/gui/fitCommands/calc/fitReplaceModule.py
index 6f0cd41e7..93ab3ec04 100644
--- a/gui/fitCommands/calc/fitReplaceModule.py
+++ b/gui/fitCommands/calc/fitReplaceModule.py
@@ -29,13 +29,13 @@ class FitReplaceModuleCommand(wx.Command):
         if newMod is None:
             return False
         # Dummy it out in case the next bit fails
-        fit.modules.toDummy(self.position)
+        fit.modules.free(self.position)
         if not newMod.fits(fit):
             pyfalog.warning('Module does not fit')
             self.Undo()
             return False
         newMod.owner = fit
-        fit.modules.toModule(self.position, newMod)
+        fit.modules.replace(self.position, newMod)
         sFit.checkStates(fit, newMod)
         eos.db.commit()
         return True
@@ -53,13 +53,13 @@ class FitReplaceModuleCommand(wx.Command):
         oldMod = self.oldModInfo.toModule()
         if oldMod is None:
             return False
-        fit.modules.toDummy(self.position)
+        fit.modules.free(self.position)
         if not oldMod.fits(fit):
             pyfalog.warning('Module does not fit')
             self.Do()
             return False
         oldMod.owner = fit
-        fit.modules.toModule(self.position, oldMod)
+        fit.modules.replace(self.position, oldMod)
         sFit.checkStates(fit, oldMod)
         eos.db.commit()
         return True
diff --git a/service/fitDeprecated.py b/service/fitDeprecated.py
index 20410541b..a103fad77 100644
--- a/service/fitDeprecated.py
+++ b/service/fitDeprecated.py
@@ -408,7 +408,7 @@ class FitDeprecated(object):
         modulesChanged = False
         for x in positions:
             if not fit.modules[x].isEmpty:
-                fit.modules.toDummy(x)
+                fit.modules.free(x)
                 modulesChanged = True
 
         # if no modules have changes, report back None
@@ -438,7 +438,7 @@ class FitDeprecated(object):
         item = eos.db.getItem(newItemID, eager=("attributes", "group.category"))
 
         # Dummy it out in case the next bit fails
-        fit.modules.toDummy(position)
+        fit.modules.free(position)
 
         try:
             m = es_Module(item)
@@ -448,7 +448,7 @@ class FitDeprecated(object):
 
         if m.fits(fit):
             m.owner = fit
-            fit.modules.toModule(position, m)
+            fit.modules.replace(position, m)
             if m.isValidState(FittingModuleState.ACTIVE):
                 m.state = FittingModuleState.ACTIVE