diff --git a/conversion_utils/jos_msds_and_properties.py b/conversion_utils/jos_msds_and_properties.py
index 5691bf4..df1f046 100644
--- a/conversion_utils/jos_msds_and_properties.py
+++ b/conversion_utils/jos_msds_and_properties.py
@@ -383,6 +383,8 @@ class Converter:
         feature_value_list = []
         lexeme_feature_map = {}
         form_feature_map = {}
+        if msd.state not in [Msd.State.FULL, Msd.State.PARTIAL]:
+            raise MsdException(f"Msd must be full or partial to be converted to Properties, but is {msd.state.name}.")
         for (index, value_char) in enumerate(value_chars, start=1):
             if (value_char != '-'):
                 feature = category.find_feature_by_position(index)