diff --git a/core/src/main/java/net/automatalib/automata/base/fast/AbstractFastMutableNondet.java b/core/src/main/java/net/automatalib/automata/base/fast/AbstractFastMutableNondet.java
index <HASH>..<HASH> 100644
--- a/core/src/main/java/net/automatalib/automata/base/fast/AbstractFastMutableNondet.java
+++ b/core/src/main/java/net/automatalib/automata/base/fast/AbstractFastMutableNondet.java
@@ -16,6 +16,7 @@
 package net.automatalib.automata.base.fast;
 
 import java.util.Collection;
+import java.util.Collections;
 import java.util.HashSet;
 import java.util.Set;
 
@@ -73,7 +74,8 @@ public abstract class AbstractFastMutableNondet<S extends AbstractFastNondetStat
     @Override
     public Collection<T> getTransitions(S state, I input) {
         int inputIdx = inputAlphabet.getSymbolIndex(input);
-        return state.getTransitions(inputIdx);
+        final Collection<T> result = state.getTransitions(inputIdx);
+        return result == null ? Collections.emptySet() : result;
     }
 
     @Override
core: fix NPE on partial NFAs

(cherry picked from commit a<I>e<I>d<I>b<I>b<I>ae4d1c8e<I>dad8d)