diff --git a/core/src/main/java/org/teavm/model/util/PhiUpdater.java b/core/src/main/java/org/teavm/model/util/PhiUpdater.java
index 40d20a840..bddddc0d9 100644
--- a/core/src/main/java/org/teavm/model/util/PhiUpdater.java
+++ b/core/src/main/java/org/teavm/model/util/PhiUpdater.java
@@ -354,11 +354,7 @@ public class PhiUpdater {
                 continue;
             }
 
-            Map<Variable, TryCatchJoint> joints = jointMap.get(tryCatch);
-            if (joints == null) {
-                joints = new HashMap<>();
-                jointMap.put(tryCatch, joints);
-            }
+            Map<Variable, TryCatchJoint> joints = jointMap.computeIfAbsent(tryCatch, k -> new HashMap<>());
             TryCatchJoint joint = joints.get(original);
             if (joint == null) {
                 joint = new TryCatchJoint();
@@ -398,7 +394,7 @@ public class PhiUpdater {
     private Variable use(Variable var) {
         Variable mappedVar = variableMap[var.getIndex()];
         if (mappedVar == null) {
-            throw new AssertionError();
+            throw new AssertionError("Variable used before definition: " + var.getIndex());
         }
         return mappedVar;
     }
diff --git a/core/src/main/java/org/teavm/vm/TeaVM.java b/core/src/main/java/org/teavm/vm/TeaVM.java
index 900d09b29..d61b5c434 100644
--- a/core/src/main/java/org/teavm/vm/TeaVM.java
+++ b/core/src/main/java/org/teavm/vm/TeaVM.java
@@ -61,6 +61,7 @@ import org.teavm.model.optimization.MethodOptimization;
 import org.teavm.model.optimization.RedundantJumpElimination;
 import org.teavm.model.optimization.UnreachableBasicBlockElimination;
 import org.teavm.model.optimization.UnusedVariableElimination;
+import org.teavm.model.util.ListingBuilder;
 import org.teavm.model.util.MissingItemsProcessor;
 import org.teavm.model.util.ModelUtils;
 import org.teavm.model.util.ProgramUtils;
@@ -478,7 +479,15 @@ public class TeaVM implements TeaVMHost, ServiceRepository {
                 do {
                     changed = false;
                     for (MethodOptimization optimization : getOptimizations()) {
-                        changed |= optimization.optimize(method, optimizedProgram);
+                        try {
+                            changed |= optimization.optimize(method, optimizedProgram);
+                        } catch (Exception e) {
+                            ListingBuilder listingBuilder = new ListingBuilder();
+                            String listing = listingBuilder.buildListing(optimizedProgram, "");
+                            System.err.println("Error optimizing program for method" + method.getReference()
+                                    + ":\n" + listing);
+                            throw new RuntimeException(e);
+                        }
                     }
                 } while (changed);