diff --git a/src/ir/Module.h b/src/ir/Module.h
index e55b8a303..08c0dcefc 100644
--- a/src/ir/Module.h
+++ b/src/ir/Module.h
@@ -29,7 +29,7 @@ namespace ir {
          *
          * @param name The name of the boolean variable to add.
          */
-		virtual uint_fast64_t addBooleanVariable(std::string const& name) = 0;
+		uint_fast64_t addBooleanVariable(std::string const& name) = 0;
 
         /*!
          * Adds an integer variable with the given name, lower and upper bound.
@@ -38,17 +38,17 @@ namespace ir {
          * @param lower The lower bound of the integer variable.
          * @param upper The upper bound of the integer variable.
          */
-		virtual uint_fast64_t addIntegerVariable(std::string const& name) = 0;
+		uint_fast64_t addIntegerVariable(std::string const& name) = 0;
         
         /*!
          * Retrieves the next free (global) index for a boolean variable.
          */
-        virtual uint_fast64_t getNextGlobalBooleanVariableIndex() = 0;
+        uint_fast64_t getNextGlobalBooleanVariableIndex() const = 0;
         
         /*!
          * Retrieves the next free (global) index for a integer variable.
          */
-        virtual uint_fast64_t getNextGlobalIntegerVariableIndex() = 0;
+        uint_fast64_t getNextGlobalIntegerVariableIndex() const = 0;
 	};
 
 /*!
diff --git a/src/parser/prismparser/VariableState.cpp b/src/parser/prismparser/VariableState.cpp
index 1bdf7b0e0..29feb025b 100644
--- a/src/parser/prismparser/VariableState.cpp
+++ b/src/parser/prismparser/VariableState.cpp
@@ -86,7 +86,7 @@ std::shared_ptr<VariableExpression> VariableState::getBooleanVariableExpression(
 	} else {
 		if (firstRun) {
 			LOG4CPLUS_TRACE(logger, "Trying to retrieve boolean variable " << name << " that was not yet created; returning dummy instead.");
-			return std::shared_ptr<VariableExpression>(nullptr);
+			return std::shared_ptr<VariableExpression>(BaseExpression::bool_, 0, name);
 		} else {
 			LOG4CPLUS_ERROR(logger, "Boolean variable " << name << " does not exist.");
             throw storm::exceptions::InvalidArgumentException() << "Boolean variable " << name << " does not exist.";
@@ -101,7 +101,7 @@ std::shared_ptr<VariableExpression> VariableState::getIntegerVariableExpression(
 	} else {
 		if (firstRun) {
 			LOG4CPLUS_TRACE(logger, "Trying to retrieve integer variable " << name << " that was not yet created; returning dummy instead.");
-			return std::shared_ptr<VariableExpression>(nullptr);
+			return std::shared_ptr<VariableExpression>(BaseExpression::int_, 0, name);
 		} else {
 			LOG4CPLUS_ERROR(logger, "Integer variable " << name << " does not exist.");
             throw storm::exceptions::InvalidArgumentException() << "Integer variable " << name << " does not exist.";