diff --git a/lib/flow_runner.ex b/lib/flow_runner.ex
index 5e137e1..4124953 100644
--- a/lib/flow_runner.ex
+++ b/lib/flow_runner.ex
@@ -123,6 +123,17 @@ defmodule FlowRunner do
     )
   end
 
+  @doc """
+  Evaluates a block or condition expression.
+
+  Runtime arithmetic in an expression can fail even when the operands are valid
+  numbers, division by zero being the canonical case, and
+  `Expression.evaluate_block!/4` surfaces that as an `ArithmeticError`. Rather than
+  let it crash the calling process, we treat it like any other unusable condition
+  value: the caller (`FlowRunner.Spec.Exit.evaluate/2`,
+  `FlowRunner.Spec.Blocks.Case.evaluate_outgoing/5`) falls through to the block's
+  default exit.
+  """
   @impl FlowRunner.Contract
   def evaluate_expression_block(expression, %Context{} = context) do
     Expression.evaluate_block!(
@@ -134,6 +145,9 @@ defmodule FlowRunner do
   rescue
     e in Expression.Error ->
       {:error, e.type, e.message}
+
+    e in ArithmeticError ->
+      {:error, :arithmetic, Exception.message(e)}
   end
 
   defdelegate fetch_resource_by_uuid(container, uuid), to: FlowRunner.Spec.Container
diff --git a/lib/flow_runner/spec/exit.ex b/lib/flow_runner/spec/exit.ex
index a8a6d78..04ce636 100644
--- a/lib/flow_runner/spec/exit.ex
+++ b/lib/flow_runner/spec/exit.ex
@@ -35,16 +35,16 @@ defmodule FlowRunner.Spec.Exit do
     test = exit.test || ""
 
     case FlowRunner.evaluate_expression_block(test, context) |> extract_value() do
-      other when not is_boolean(other) ->
+      {:error, reason, bad_parts} ->
         Logger.info(
-          "Expression '#{exit.test}' returned #{inspect(other)} when expecting a boolean last_block_uuid=#{context.last_block_uuid}"
+          "Expression '#{exit.test}' returned parsing error #{inspect(reason)} when hit #{inspect(bad_parts)} last_block_uuid=#{context.last_block_uuid}"
         )
 
         false
 
-      {:error, reason, bad_parts} ->
+      other when not is_boolean(other) ->
         Logger.info(
-          "Expression '#{exit.test}' returned parsing error #{inspect(reason)} when hit #{inspect(bad_parts)} last_block_uuid=#{context.last_block_uuid}"
+          "Expression '#{exit.test}' returned #{inspect(other)} when expecting a boolean last_block_uuid=#{context.last_block_uuid}"
         )
 
         false
diff --git a/test/block_test.exs b/test/block_test.exs
index f889e56..dc33718 100644
--- a/test/block_test.exs
+++ b/test/block_test.exs
@@ -152,6 +152,28 @@ defmodule BlockTest do
              Block.evaluate_exits(block, context)
   end
 
+  test "an exit whose test raises a runtime arithmetic error falls through to the default exit" do
+    context = %Context{
+      vars: %{"score" => 10, "total" => 0}
+    }
+
+    block = %Block{
+      exits: [
+        %Exit{
+          uuid: "b586afa7-0097-4805-9951-f6d3156c08db",
+          test: "score / total > 1"
+        },
+        %Exit{
+          uuid: "cbf4382f-0ce0-426b-9a3c-40cbe84dd081",
+          default: true
+        }
+      ]
+    }
+
+    assert {:ok, %Exit{uuid: "cbf4382f-0ce0-426b-9a3c-40cbe84dd081"}} =
+             Block.evaluate_exits(block, context)
+  end
+
   test "fetch default block" do
     # 1. With NO default exit
     context = %Context{
diff --git a/test/flow_runner/expression_mode_test.exs b/test/flow_runner/expression_mode_test.exs
index 0ffcbdf..232dff7 100644
--- a/test/flow_runner/expression_mode_test.exs
+++ b/test/flow_runner/expression_mode_test.exs
@@ -48,6 +48,14 @@ defmodule FlowRunner.ExpressionModeTest do
       assert FlowRunner.evaluate_expression_block("age", context(%{"age" => "30"}, :v3)) ==
                "30"
     end
+
+    test "a runtime arithmetic failure returns an :arithmetic error instead of raising" do
+      assert {:error, :arithmetic, _message} =
+               FlowRunner.evaluate_expression_block(
+                 "score / total",
+                 context(%{"score" => 10, "total" => 0}, :v2)
+               )
+    end
   end
 
   describe "evaluate_expression_as_string!/2" do