diff --git a/src/storm/builder/ExplicitModelBuilder.cpp b/src/storm/builder/ExplicitModelBuilder.cpp
index b80825d1d..9ea3a6c5b 100644
--- a/src/storm/builder/ExplicitModelBuilder.cpp
+++ b/src/storm/builder/ExplicitModelBuilder.cpp
@@ -41,10 +41,18 @@ namespace storm {
         template<typename StateType>
         StateType ExplicitStateLookup<StateType>::lookup(std::map<storm::expressions::Variable, storm::expressions::Expression> const& stateDescription) const {
             auto cs = storm::generator::createCompressedState(this->varInfo, stateDescription, true);
-            STORM_LOG_THROW(stateToId.contains(cs), storm::exceptions::IllegalArgumentException, "State unknown.");
+            //TODO search once
+            if (!stateToId.contains(cs)) {
+                return static_cast<StateType>(this->size());
+            }
             return this->stateToId.getValue(cs);
         }
 
+        template<typename StateType>
+        uint64_t ExplicitStateLookup<StateType>::size() const {
+            return this->stateToId.size();
+        }
+
         template <typename ValueType, typename RewardModelType, typename StateType>
         ExplicitModelBuilder<ValueType, RewardModelType, StateType>::Options::Options() : explorationOrder(storm::settings::getModule<storm::settings::modules::BuildSettings>().getExplorationOrder()) {
             // Intentionally left empty.
diff --git a/src/storm/builder/ExplicitModelBuilder.h b/src/storm/builder/ExplicitModelBuilder.h
index f33b37ba3..1fb451889 100644
--- a/src/storm/builder/ExplicitModelBuilder.h
+++ b/src/storm/builder/ExplicitModelBuilder.h
@@ -53,7 +53,16 @@ namespace storm {
                 // intentionally left empty.
             }
 
+            /**
+             * Lookup state
+             * @param stateDescription A map describing the state
+             * @return The id of the state, or size() when no state is found
+             */
             StateType lookup(std::map<storm::expressions::Variable, storm::expressions::Expression> const& stateDescription) const;
+            /**
+             * How many states have been stored?
+             */
+            uint64_t size() const;
 
         private:
             VariableInformation varInfo;