diff --git a/cineast-api/src/main/java/org/vitrivr/cineast/api/rest/handlers/actions/bool/FindDistinctElementsByColumnPostHandler.java b/cineast-api/src/main/java/org/vitrivr/cineast/api/rest/handlers/actions/bool/FindDistinctElementsByColumnPostHandler.java
index 64f45670a..9038e4aff 100644
--- a/cineast-api/src/main/java/org/vitrivr/cineast/api/rest/handlers/actions/bool/FindDistinctElementsByColumnPostHandler.java
+++ b/cineast-api/src/main/java/org/vitrivr/cineast/api/rest/handlers/actions/bool/FindDistinctElementsByColumnPostHandler.java
@@ -16,6 +16,7 @@
 import org.vitrivr.cineast.api.messages.result.DistinctElementsResult;
 import org.vitrivr.cineast.api.rest.handlers.interfaces.ParsingPostRestHandler;
 import org.vitrivr.cineast.core.data.providers.primitive.PrimitiveTypeProvider;
+import org.vitrivr.cineast.core.data.providers.primitive.ProviderDataType;
 import org.vitrivr.cineast.core.db.DBSelector;
 import org.vitrivr.cineast.standalone.config.Config;
 
@@ -48,7 +49,7 @@ public DistinctElementsResult performPost(ColumnSpecification specification, Con
     }
     StopWatch watch = StopWatch.createStarted();
     selector.open(specification.table());
-    distinct = selector.getUniqueValues(specification.column()).stream().map(PrimitiveTypeProvider::getString).collect(Collectors.toList());
+    distinct = selector.getUniqueValues(specification.column()).stream().filter(p -> p.getType() != ProviderDataType.UNKNOWN).map(PrimitiveTypeProvider::getString).collect(Collectors.toList());
     cache.put(specification.table() + specification.column(), distinct);
     LOGGER.trace("Retrieved unique values for {} in {} ms", specification.table() + "." + specification.column(), watch.getTime(TimeUnit.MILLISECONDS));
     return new DistinctElementsResult("", distinct);
diff --git a/cineast-core/src/main/java/org/vitrivr/cineast/core/data/providers/primitive/NothingProvider.java b/cineast-core/src/main/java/org/vitrivr/cineast/core/data/providers/primitive/NothingProvider.java
index 2cd208b9f..28bfcc624 100644
--- a/cineast-core/src/main/java/org/vitrivr/cineast/core/data/providers/primitive/NothingProvider.java
+++ b/cineast-core/src/main/java/org/vitrivr/cineast/core/data/providers/primitive/NothingProvider.java
@@ -14,4 +14,4 @@ public String toString() {
     return "NothingProvider";
   }
 
-}
\ No newline at end of file
+}
diff --git a/cineast-core/src/main/java/org/vitrivr/cineast/core/data/providers/primitive/StringProvider.java b/cineast-core/src/main/java/org/vitrivr/cineast/core/data/providers/primitive/StringProvider.java
index 13c558f94..2a575c9d6 100644
--- a/cineast-core/src/main/java/org/vitrivr/cineast/core/data/providers/primitive/StringProvider.java
+++ b/cineast-core/src/main/java/org/vitrivr/cineast/core/data/providers/primitive/StringProvider.java
@@ -3,7 +3,7 @@
 public interface StringProvider {
 
   default String getString() {
-    throw new UnsupportedOperationException("No string value specified");
+    throw new UnsupportedOperationException("No string value specified: " + this);
   }
 
 }
diff --git a/cineast-core/src/main/java/org/vitrivr/cineast/core/features/abstracts/BooleanRetriever.java b/cineast-core/src/main/java/org/vitrivr/cineast/core/features/abstracts/BooleanRetriever.java
index 54420bbda..10f0899f3 100644
--- a/cineast-core/src/main/java/org/vitrivr/cineast/core/features/abstracts/BooleanRetriever.java
+++ b/cineast-core/src/main/java/org/vitrivr/cineast/core/features/abstracts/BooleanRetriever.java
@@ -89,6 +89,7 @@ public List<ScoreElement> getSimilar(SegmentContainer sc, ReadableQueryConfig qc
 
     if (relevantExpressions.isEmpty()) {
       LOGGER.debug("No relevant expressions in {} for query {}", this.getClass().getSimpleName(), sc.toString());
+      LOGGER.debug("In this class: {}, {}", this.getSupportedOperators(), this.getAttributes());
       return Collections.emptyList();
     }