diff --git a/webbpsf/tests/test_nirspec.py b/webbpsf/tests/test_nirspec.py
index 6575839c..c3cf866e 100644
--- a/webbpsf/tests/test_nirspec.py
+++ b/webbpsf/tests/test_nirspec.py
@@ -61,6 +61,7 @@ def test_mode_switch():
     nrs.mode = 'IFU'
     assert 'IFU' in nrs.aperturename
     assert nrs.band == 'PRISM/CLEAR'
+    assert nrs.image_mask is None
 
     # check switch of which IFU band
     nrs.disperser = 'G395H'
diff --git a/webbpsf/webbpsf_core.py b/webbpsf/webbpsf_core.py
index fd72191c..94c081c7 100644
--- a/webbpsf/webbpsf_core.py
+++ b/webbpsf/webbpsf_core.py
@@ -2105,8 +2105,6 @@ def filter(self, value):
 
     def _validate_config(self, **kwargs):
         """Validate instrument config for MIRI"""
-        if self.filter.startswith('MRS-IFU'):
-            raise NotImplementedError('The MIRI MRS is not yet implemented.')
         return super(MIRI, self)._validate_config(**kwargs)
 
     def _addAdditionalOptics(self, optsys, oversample=2):
@@ -3142,8 +3140,6 @@ def __init__(self):
         self._si_wfe_class = optics.NIRSpecFieldDependentAberration  # note we end up adding 2 instances of this.
 
     def _validate_config(self, **kwargs):
-        if self.filter.startswith('IFU'):
-            raise NotImplementedError('The NIRSpec IFU is not yet implemented.')
         return super(NIRSpec, self)._validate_config(**kwargs)
 
     def _addAdditionalOptics(self, optsys, oversample=2):
@@ -3259,6 +3255,9 @@ def aperturename(self, value):
                     if self._disperser is None:
                         self.disperser = 'PRISM'  # Set some default spectral mode
                         self.filter = 'CLEAR'
+                    if self.image_mask not in ['IFU', None]:
+                        _log.info("The currently-selected image mask (slit) is not compatible with IFU mode. Setting image_mask=None")
+                        self.image_mask = None
                 else:
                     self._mode = 'imaging' # More to implement here later!