diff --git a/README.md b/README.md
index 1cc04c9..046b18d 100644
--- a/README.md
+++ b/README.md
@@ -180,6 +180,18 @@ end
 
 `data_errors` returns a `{ fact_incomplete_definition: ... }` sentinel that downstream facts can detect.
 
+#### A fact must never resolve to `nil`
+
+Every fact has to return a real value or signal an error (`data_errors` / an error
+hash). `nil` is not a valid result: it leaves the facts downstream unable to tell a
+missing value apart from a satisfied one, so the missing dependency propagates
+silently. The evaluator enforces this — a fact that resolves to `nil` raises
+`FactGraph::NullFactError` naming the offending fact. In practice this catches two
+mistakes: an `allow_unmet_dependencies` fact whose branch returns `nil` instead of
+`data_errors`, and an input fact whose validator permits `nil` (leave the `nil`
+case to an error instead of passing it through). Note that `false` is a valid
+value and does not raise.
+
 `must_match` is a convenience wrapper that runs a pattern-match block and falls back to `data_errors` on `NoMatchingPatternError`:
 
 ```ruby
diff --git a/lib/fact_graph.rb b/lib/fact_graph.rb
index 6639f0b..9c3de0f 100644
--- a/lib/fact_graph.rb
+++ b/lib/fact_graph.rb
@@ -10,6 +10,10 @@
 module FactGraph
   class ValidationError < StandardError; end
 
+  # Raised when a fact resolves to nil instead of a value or an unmet-dependency
+  # error hash — nil leaves dependents unable to tell "missing" from "satisfied".
+  class NullFactError < StandardError; end
+
   class Graph
     @graph_registry = []
 
diff --git a/lib/fact_graph/fact.rb b/lib/fact_graph/fact.rb
index dd72557..d7d8cc8 100644
--- a/lib/fact_graph/fact.rb
+++ b/lib/fact_graph/fact.rb
@@ -147,6 +147,7 @@ def call(input, results)
     results[module_name] ||= {}
 
     if !resolver.respond_to?(:call)
+      forbid_nil!(resolver)
       results[module_name][name] = resolver
       return resolver
     end
@@ -195,11 +196,25 @@ def call(input, results)
       resolved_errors = data_errors
     end
 
+    value = resolved_errors || data.instance_exec(&resolver)
+    forbid_nil!(value)
+
     if per_entity
       results[module_name][name] ||= {}
-      results[module_name][name][entity_id] = resolved_errors || data.instance_exec(&resolver)
+      results[module_name][name][entity_id] = value
     else
-      results[module_name][name] = resolved_errors || data.instance_exec(&resolver)
+      results[module_name][name] = value
     end
   end
+
+  private
+
+  def forbid_nil!(value)
+    return unless value.nil?
+
+    fact_ref = per_entity ? ":#{module_name}/:#{name} (entity #{entity_id})" : ":#{module_name}/:#{name}"
+    raise FactGraph::NullFactError,
+      "Fact #{fact_ref} resolved to nil. A fact must return a value or signal an " \
+      "unmet dependency (return the error hash / data_errors), never nil."
+  end
 end
diff --git a/spec/fact_graph/fact_spec.rb b/spec/fact_graph/fact_spec.rb
index bdb3e24..0203275 100644
--- a/spec/fact_graph/fact_spec.rb
+++ b/spec/fact_graph/fact_spec.rb
@@ -175,6 +175,79 @@
     end
   end
 
+  describe "null fact detection" do
+    before { FactGraph::Graph.graph_registry = [] }
+
+    def evaluate_single_fact(module_name, fact_name, input = {})
+      graph = FactGraph::Graph.prepare_fact_objects(input)
+      graph[module_name][fact_name].call(input, {})
+    end
+
+    it "raises when a resolver proc returns nil" do
+      Class.new(FactGraph::Graph) do
+        in_module :leaks do
+          fact :leaky do
+            proc { nil }
+          end
+        end
+      end
+      expect { evaluate_single_fact(:leaks, :leaky) }
+        .to raise_error(FactGraph::NullFactError, /:leaks\/:leaky resolved to nil/)
+    end
+
+    it "raises when an allow_unmet_dependencies fact returns nil on its unmet path" do
+      Class.new(FactGraph::Graph) do
+        in_module :leaks do
+          fact :missing_input do
+            input :missing_input, value: :integer
+          end
+          fact :leaky, allow_unmet_dependencies: true do
+            dependency :missing_input
+            proc { nil }
+          end
+        end
+      end
+      expect { evaluate_single_fact(:leaks, :leaky) }
+        .to raise_error(FactGraph::NullFactError, /:leaks\/:leaky resolved to nil/)
+    end
+
+    it "raises when an input fact passes through a nil value" do
+      Class.new(FactGraph::Graph) do
+        in_module :leaks do
+          fact :optional do
+            input [:wrapper, :optional]
+          end
+        end
+      end
+      expect { evaluate_single_fact(:leaks, :optional, {wrapper: {optional: nil}}) }
+        .to raise_error(FactGraph::NullFactError, /:leaks\/:optional resolved to nil/)
+    end
+
+    it "names the entity for a per-entity fact that returns nil" do
+      Class.new(FactGraph::Graph) do
+        in_module :leaks do
+          fact :leaky, per_entity: :items do
+            proc { nil }
+          end
+        end
+      end
+      expect { FactGraph::Evaluator.evaluate({items: [{}]}) }
+        .to raise_error(FactGraph::NullFactError, /:leaks\/:leaky \(entity 0\)/)
+    end
+
+    it "does not raise when a fact resolves to false" do
+      Class.new(FactGraph::Graph) do
+        in_module :ok do
+          fact :falsey do
+            proc { false }
+          end
+        end
+      end
+      result = evaluate_single_fact(:ok, :falsey)
+      expect(result).to eq false
+    end
+  end
+
   describe "short-form input with no value predicates" do
     before do
       FactGraph::Graph.graph_registry = []