diff --git a/alib2data/src/automaton/PDA/RealTimeHeightDeterministicDPDA.cpp b/alib2data/src/automaton/PDA/RealTimeHeightDeterministicDPDA.cpp
index dfb57f223107a7f7f186342c5154e9b3e090ab30..866c9c57b9ddc38507bdea1ef49b92114d447f98 100644
--- a/alib2data/src/automaton/PDA/RealTimeHeightDeterministicDPDA.cpp
+++ b/alib2data/src/automaton/PDA/RealTimeHeightDeterministicDPDA.cpp
@@ -113,7 +113,7 @@ bool RealTimeHeightDeterministicDPDA::addReturnTransition(State from, std::varia
 		throw AutomatonException("State \"" + (std::string) to.getName() + "\" doesn't exist.");
 	}
 
-	if (getPushdownStoreAlphabet().count(pop)) {
+	if (!getPushdownStoreAlphabet().count(pop)) {
 		throw AutomatonException("Pushdown store symbol \"" + (std::string) pop + "\" doesn't exist.");
 	}