diff --git a/runtime/Java/src/org/antlr/v4/runtime/atn/ATNSimulator.java b/runtime/Java/src/org/antlr/v4/runtime/atn/ATNSimulator.java
index a2a03da16..e24d5c82d 100644
--- a/runtime/Java/src/org/antlr/v4/runtime/atn/ATNSimulator.java
+++ b/runtime/Java/src/org/antlr/v4/runtime/atn/ATNSimulator.java
@@ -349,6 +349,7 @@ public abstract class ATNSimulator {
 		return c==65535 ? -1 : c;
 	}
 
+	@NotNull
 	public static Transition edgeFactory(@NotNull ATN atn,
 										 int type, int src, int trg,
 										 int arg1, int arg2, int arg3,
@@ -372,11 +373,12 @@ public abstract class ATNSimulator {
 			case Transition.NOT_SET : return new NotSetTransition(target, sets.get(arg1));
 			case Transition.WILDCARD : return new WildcardTransition(target);
 		}
-		return null;
+
+		throw new IllegalArgumentException("The specified transition type is not valid.");
 	}
 
 	public static ATNState stateFactory(int type, int stateNumber) {
-		ATNState s = null;
+		ATNState s;
 		switch (type) {
 			case ATNState.INVALID_TYPE: return null;
 			case ATNState.BASIC : s = new ATNState(); break;
@@ -392,9 +394,10 @@ public abstract class ATNSimulator {
 			case ATNState.PLUS_LOOP_BACK : s = new PlusLoopbackState(); break;
 			case ATNState.LOOP_END : s = new LoopEndState(); break;
             default :
-                System.err.println("invalid state type in ATN deserialization: "+type+" for state "+stateNumber);
-                break;
+				String message = String.format("The specified state type %d for state %d is not valid.", type, stateNumber);
+				throw new IllegalArgumentException(message);
 		}
+
 		s.stateNumber = stateNumber;
 		return s;
 	}
diff --git a/tool/src/org/antlr/v4/automata/ParserATNFactory.java b/tool/src/org/antlr/v4/automata/ParserATNFactory.java
index c880090d3..c17cb9ca3 100644
--- a/tool/src/org/antlr/v4/automata/ParserATNFactory.java
+++ b/tool/src/org/antlr/v4/automata/ParserATNFactory.java
@@ -77,6 +77,7 @@ import org.antlr.v4.tool.ast.QuantifierAST;
 import org.antlr.v4.tool.ast.TerminalAST;
 
 import java.lang.reflect.Constructor;
+import java.lang.reflect.InvocationTargetException;
 import java.util.Collection;
 import java.util.List;
 
@@ -621,9 +622,13 @@ public class ParserATNFactory implements ATNFactory {
 		return t;
 	}
 
-	// TODO (sam): should we allow this to throw an exception instead of returning null?
-	@Nullable
+	@NotNull
 	public <T extends ATNState> T newState(@NotNull Class<T> nodeType, GrammarAST node) {
+		if (!ATNState.class.isAssignableFrom(nodeType)) {
+			throw new IllegalArgumentException(String.format("%s is not a %s.", nodeType.getName(), ATNState.class.getName()));
+		}
+
+		Exception cause;
 		try {
 			Constructor<T> ctor = nodeType.getConstructor();
 			T s = ctor.newInstance();
@@ -631,11 +636,22 @@ public class ParserATNFactory implements ATNFactory {
 			else s.setRuleIndex(currentRule.index);
 			atn.addState(s);
 			return s;
+		} catch (InstantiationException ex) {
+			cause = ex;
+		} catch (IllegalAccessException ex) {
+			cause = ex;
+		} catch (IllegalArgumentException ex) {
+			cause = ex;
+		} catch (InvocationTargetException ex) {
+			cause = ex;
+		} catch (NoSuchMethodException ex) {
+			cause = ex;
+		} catch (SecurityException ex) {
+			cause = ex;
 		}
-		catch (Exception e) {
-			ErrorManager.internalError("can't create ATN node: "+nodeType.getName(), e);
-		}
-		return null;
+
+		String message = String.format("Could not create %s of type %s.", ATNState.class.getName(), nodeType.getName());
+		throw new UnsupportedOperationException(message, cause);
 	}
 
 	@NotNull