fix

diff --git a/src/passes/StackIR.cpp b/src/passes/StackIR.cpp
index c5b1524..39fbdfe 100644
--- a/src/passes/StackIR.cpp
+++ b/src/passes/StackIR.cpp

@@ -251,12 +251,13 @@
         // rid of it, which might not be worth it.
         // TODO: the case of 2 seems optimizable, by replacing the old value
         //       with a drop.
+        assert(!valueStack.empty());
         auto droppedIndex = valueStack.back();
         valueStack.pop_back();
         auto* droppedInst = insts[droppedIndex];
         // TODO: handle drops of control flow elements like blocks, which show
         //       up as multiple insts.
-        if (!isControlFlow(droppedInst)) {
+        if (droppedInst && !isControlFlow(droppedInst)) {
           auto consumedByDropped = getNumConsumedValues(droppedInst);
           if (consumedByDropped < 2) {
             EffectAnalyzer effects(passOptions, module->features);
@@ -279,10 +280,13 @@
       //       to handle unreachable code here - it's ok to pop multiple values
       //       there even if the stack is at size 0.
       while (consumed > 0) {
+        assert(!valueStack.empty());
         valueStack.pop_back();
         consumed--;
       }
-      if (isBegin(inst) && inst->type.isConcrete()) {
+      // Push a value on the stack if there is one (note the convention in stack
+      // IR where the end of a block etc. represents the value).
+      if (inst->type.isConcrete() && isEnd(inst)) {
         valueStack.push_back(i);
       }
     }
@@ -358,6 +362,10 @@
     return inst->op == StackInst::Basic || isControlFlowBegin(inst);
   }
 
+  bool isEnd(StackInst* inst) {
+    return inst->op == StackInst::Basic || isControlFlowEnd(inst);
+  }
+
   // Remove the instruction at index i. If the instruction
   // is control flow, and so has been expanded to multiple
   // instructions, remove them as well.

diff --git a/src/wasm/wasm-stack.cpp b/src/wasm/wasm-stack.cpp
index 2e4cd43..15e457d 100644
--- a/src/wasm/wasm-stack.cpp
+++ b/src/wasm/wasm-stack.cpp

@@ -1753,7 +1753,7 @@
       stackType = Type::none;
     } else if (op != StackInst::BlockEnd && op != StackInst::IfEnd &&
                op != StackInst::LoopEnd && op != StackInst::TryEnd) {
-      // If a concrete type is returned, we mark the end of the construct has
+      // If a concrete type is returned, we mark the end of the construct as
       // having that type (as it is pushed to the value stack at that point),
       // other parts are marked as none).
       stackType = Type::none;

diff --git a/test/passes/generate-stack-ir_optimize-stack-ir_print-stack-ir_all-features.txt b/test/passes/generate-stack-ir_optimize-stack-ir_print-stack-ir_all-features.txt
index bb031c4..394cdac 100644
--- a/test/passes/generate-stack-ir_optimize-stack-ir_print-stack-ir_all-features.txt
+++ b/test/passes/generate-stack-ir_optimize-stack-ir_print-stack-ir_all-features.txt

@@ -1,6 +1,7 @@
 (module
  (type $none_=>_none (func))
  (type $i32_=>_none (func (param i32)))
+ (type $none_=>_i64 (func (result i64)))
  (event $e0 (attr 0) (param i32))
  (func $eh (; 0 ;)
   (local $exn exnref)
@@ -18,10 +19,14 @@
    drop
   end
  )
+ (func $drop_unreachable_block (; 1 ;) (result i64)
+  unreachable
+ )
 )
 (module
  (type $none_=>_none (func))
  (type $i32_=>_none (func (param i32)))
+ (type $none_=>_i64 (func (result i64)))
  (event $e0 (attr 0) (param i32))
  (func $eh (; 0 ;) (; has Stack IR ;)
   (local $exn exnref)
@@ -45,4 +50,12 @@
    )
   )
  )
+ (func $drop_unreachable_block (; 1 ;) (; has Stack IR ;) (result i64)
+  (drop
+   (block $label$1 (result i32)
+    (unreachable)
+   )
+  )
+  (i64.const 1)
+ )
 )

diff --git a/test/passes/generate-stack-ir_optimize-stack-ir_print-stack-ir_all-features.wast b/test/passes/generate-stack-ir_optimize-stack-ir_print-stack-ir_all-features.wast
index c355a2b..c9fbf8b 100644
--- a/test/passes/generate-stack-ir_optimize-stack-ir_print-stack-ir_all-features.wast
+++ b/test/passes/generate-stack-ir_optimize-stack-ir_print-stack-ir_all-features.wast

@@ -1,6 +1,5 @@
 (module
   (event $e0 (attr 0) (param i32))
-
   (func $eh (local $exn exnref)
     (try
       (throw $e0 (i32.const 0))
@@ -16,4 +15,12 @@
       )
     )
   )
+  (func $drop_unreachable_block (result i64)
+    (drop
+      (block $label$1 (result i32)
+        (unreachable)
+      )
+    )
+    (i64.const 1)
+  )
 )