diff --git a/lib/permittable/json_schema.rb b/lib/permittable/json_schema.rb
index 0d29678..7bba457 100644
--- a/lib/permittable/json_schema.rb
+++ b/lib/permittable/json_schema.rb
@@ -114,8 +114,16 @@ def nullify!(schema, field)
       schema
     end
 
+    # deep_dup, not dup: .freeze is shallow, so the Array nested in an entry
+    # like :decimal ("type" => %w[string number]) stays live inside the frozen
+    # top-level Hash, and a shallow .dup would hand every :decimal field's
+    # exported schema that SAME Array. Nothing in this file mutates it in
+    # place (nullify! rebinds "type" to a new Array), but the exported
+    # document is caller-owned data, and a caller appending to it would
+    # otherwise silently rewrite the constant for every schema exported
+    # afterward in the process.
     def scalar_schema(field)
-      schema = SCALAR_SCHEMAS.fetch(field[:type]).dup
+      schema = SCALAR_SCHEMAS.fetch(field[:type]).deep_dup
       apply_format_name!(schema, field)
       apply_in!(schema, field)
       apply_string_bounds!(schema, field)
@@ -151,7 +159,11 @@ def array_schema(field, unknown:)
       min, max = length_bounds(field[:length])
       schema["minItems"] = min if min
       schema["maxItems"] = max if max
-      schema["items"] = field[:fields] ? object(field[:fields], unknown: unknown) : SCALAR_SCHEMAS.fetch(field[:of]).dup
+      # deep_dup here for the same reason as scalar_schema: a scalar `of:`
+      # otherwise hands every array field the SAME nested Array/Hash from
+      # SCALAR_SCHEMAS.
+      schema["items"] =
+        field[:fields] ? object(field[:fields], unknown: unknown) : SCALAR_SCHEMAS.fetch(field[:of]).deep_dup
       schema
     end
 
diff --git a/spec/json_schema_spec.rb b/spec/json_schema_spec.rb
index 1b9d964..081ba4b 100644
--- a/spec/json_schema_spec.rb
+++ b/spec/json_schema_spec.rb
@@ -42,6 +42,19 @@ def quietly
       expect(props["day"]).to eq("type" => "string", "format" => "date")
       expect(props["at"]).to eq("type" => "string", "format" => "date-time")
     end
+
+    it "deep-dups a SCALAR_SCHEMAS entry, so mutating one field's exported type array never leaks " \
+       "into the frozen constant or any other field's schema" do
+      type_array = property("d") { optional :d, :decimal }["type"]
+      expect(type_array).not_to equal(Permittable::JsonSchema::SCALAR_SCHEMAS[:decimal]["type"])
+
+      type_array << "null"
+
+      expect(Permittable::JsonSchema::SCALAR_SCHEMAS[:decimal]["type"]).to eq(%w[string number])
+      expect(property("d2") { optional :d2, :decimal }["type"]).to eq(%w[string number])
+    ensure
+      Permittable::JsonSchema::SCALAR_SCHEMAS[:decimal]["type"].delete("null")
+    end
   end
 
   describe "required and absence semantics" do
@@ -649,6 +662,19 @@ def server_accepts?(field_rule, value)
       expect(items["required"]).to eq(%w[sku quantity])
       expect(items["properties"]["quantity"]).to include("minimum" => 1, "maximum" => 99)
     end
+
+    it "deep-dups a SCALAR_SCHEMAS entry for `of:`, so mutating one array field's items type never leaks " \
+       "into the frozen constant or any other field's schema" do
+      items_type = property("ds") { array :ds, of: :decimal }["items"]["type"]
+      expect(items_type).not_to equal(Permittable::JsonSchema::SCALAR_SCHEMAS[:decimal]["type"])
+
+      items_type << "null"
+
+      expect(Permittable::JsonSchema::SCALAR_SCHEMAS[:decimal]["type"]).to eq(%w[string number])
+      expect(property("ds2") { array :ds2, of: :decimal }["items"]["type"]).to eq(%w[string number])
+    ensure
+      Permittable::JsonSchema::SCALAR_SCHEMAS[:decimal]["type"].delete("null")
+    end
   end
 
   describe "root:" do