diff --git a/argument_coercion_test.go b/argument_coercion_test.go
new file mode 100644
index 0000000..104c9d5
--- /dev/null
+++ b/argument_coercion_test.go
@@ -0,0 +1,137 @@
+package graphql_test
+
+import (
+	"encoding/json"
+	"sort"
+	"testing"
+
+	"github.com/tailor-platform/graphql"
+	"github.com/tailor-platform/graphql/testutil"
+)
+
+// Serialises p.Args so tests can tell "absent", "null", and "value" apart.
+func probeArgs(p graphql.ResolveParams) (interface{}, error) {
+	keys := make([]string, 0, len(p.Args))
+	for k := range p.Args {
+		keys = append(keys, k)
+	}
+	sort.Strings(keys)
+	out := map[string]interface{}{"keys": keys}
+	for _, k := range keys {
+		v := p.Args[k]
+		if v == nil {
+			out[k] = "null"
+		} else {
+			out[k] = v
+		}
+	}
+	b, _ := json.Marshal(out)
+	return string(b), nil
+}
+
+var coercionProbeInputObject = graphql.NewInputObject(graphql.InputObjectConfig{
+	Name: "CoercionProbeInput",
+	Fields: graphql.InputObjectConfigFieldMap{
+		"a": &graphql.InputObjectFieldConfig{Type: graphql.String},
+		"b": &graphql.InputObjectFieldConfig{Type: graphql.String},
+	},
+})
+
+var coercionProbeType = graphql.NewObject(graphql.ObjectConfig{
+	Name: "CoercionProbeQuery",
+	Fields: graphql.Fields{
+		"probe": &graphql.Field{
+			Type: graphql.String,
+			Args: graphql.FieldConfigArgument{
+				"a": &graphql.ArgumentConfig{Type: graphql.String},
+				"b": &graphql.ArgumentConfig{Type: graphql.String},
+			},
+			Resolve: probeArgs,
+		},
+		"probeObject": &graphql.Field{
+			Type: graphql.String,
+			Args: graphql.FieldConfigArgument{
+				"input": &graphql.ArgumentConfig{Type: coercionProbeInputObject},
+			},
+			Resolve: func(p graphql.ResolveParams) (interface{}, error) {
+				obj, _ := p.Args["input"].(map[string]interface{})
+				keys := make([]string, 0, len(obj))
+				for k := range obj {
+					keys = append(keys, k)
+				}
+				sort.Strings(keys)
+				b, _ := json.Marshal(map[string]interface{}{
+					"keys": keys,
+					"obj":  obj,
+				})
+				return string(b), nil
+			},
+		},
+	},
+})
+
+var coercionProbeSchema, _ = graphql.NewSchema(graphql.SchemaConfig{Query: coercionProbeType})
+
+func runProbe(t *testing.T, field, doc string, vars map[string]interface{}, want string) {
+	t.Helper()
+	parsed := testutil.TestParse(t, doc)
+	result := testutil.TestExecute(t, graphql.ExecuteParams{
+		Schema: coercionProbeSchema,
+		AST:    parsed,
+		Args:   vars,
+	})
+	if len(result.Errors) > 0 {
+		t.Fatalf("unexpected errors: %v", result.Errors)
+	}
+	data, _ := result.Data.(map[string]interface{})
+	got, _ := data[field].(string)
+	if got != want {
+		t.Fatalf("probe mismatch\n  got:  %s\n  want: %s", got, want)
+	}
+}
+
+func TestArgumentCoercion_ScalarVariable_PreservesThreeStates(t *testing.T) {
+	doc := `query Probe($a: String, $b: String) { probe(a: $a, b: $b) }`
+
+	t.Run("variable omitted -> argument absent", func(t *testing.T) {
+		runProbe(t, "probe", doc,
+			map[string]interface{}{"a": "x"},
+			`{"a":"x","keys":["a"]}`)
+	})
+	t.Run("variable explicitly null -> argument present as null", func(t *testing.T) {
+		runProbe(t, "probe", doc,
+			map[string]interface{}{"a": "x", "b": nil},
+			`{"a":"x","b":"null","keys":["a","b"]}`)
+	})
+	t.Run("variable with value -> argument present with value", func(t *testing.T) {
+		runProbe(t, "probe", doc,
+			map[string]interface{}{"a": "x", "b": "y"},
+			`{"a":"x","b":"y","keys":["a","b"]}`)
+	})
+}
+
+func TestArgumentCoercion_InputObjectVariable_PreservesThreeStates(t *testing.T) {
+	doc := `query Probe($a: String, $b: String) { probeObject(input: {a: $a, b: $b}) }`
+
+	t.Run("nested variable omitted -> field absent in object", func(t *testing.T) {
+		runProbe(t, "probeObject", doc,
+			map[string]interface{}{"a": "x"},
+			`{"keys":["a"],"obj":{"a":"x"}}`)
+	})
+	t.Run("nested variable explicitly null -> field present as null", func(t *testing.T) {
+		runProbe(t, "probeObject", doc,
+			map[string]interface{}{"a": "x", "b": nil},
+			`{"keys":["a","b"],"obj":{"a":"x","b":null}}`)
+	})
+	t.Run("nested variable with value -> field present with value", func(t *testing.T) {
+		runProbe(t, "probeObject", doc,
+			map[string]interface{}{"a": "x", "b": "y"},
+			`{"keys":["a","b"],"obj":{"a":"x","b":"y"}}`)
+	})
+}
+
+func TestArgumentCoercion_InputObjectLiteral_OmittedFieldStaysAbsent(t *testing.T) {
+	doc := `{ probeObject(input: {a: "x"}) }`
+	runProbe(t, "probeObject", doc, nil,
+		`{"keys":["a"],"obj":{"a":"x"}}`)
+}
diff --git a/values.go b/values.go
index 4ed4b46..3a49bc1 100644
--- a/values.go
+++ b/values.go
@@ -27,9 +27,12 @@ func getVariableValues(
 			continue
 		}
 		varName := defAST.Variable.Name.Value
-		if varValue, err := getVariableValue(schema, defAST, inputs[varName]); err != nil {
+		input, provided := inputs[varName]
+		varValue, err := getVariableValue(schema, defAST, input)
+		if err != nil {
 			return values, err
-		} else {
+		}
+		if provided || defAST.DefaultValue != nil {
 			values[varName] = varValue
 		}
 	}
@@ -50,28 +53,33 @@ func getArgumentValues(
 	}
 	results := map[string]interface{}{}
 	for _, argDef := range argDefs {
-		var (
-			tmp         interface{}
-			value       ast.Value
-			isUndefined bool
-		)
-		if tmpValue, ok := argASTMap[argDef.PrivateName]; ok {
-			value = tmpValue.Value
-		} else {
-			isUndefined = true
-		}
-		if tmp = valueFromAST(value, argDef.Type, variableValues); isNullish(tmp) {
+		var value ast.Value
+		argAST, ok := argASTMap[argDef.PrivateName]
+		if ok {
+			value = argAST.Value
+		}
+		isUndefined := !ok || isUnprovidedVariable(value, variableValues)
+		tmp := valueFromAST(value, argDef.Type, variableValues)
+		if isNullish(tmp) {
 			tmp = argDef.DefaultValue
 		}
-		if !isUndefined && tmp == nil {
-			results[argDef.PrivateName] = nil
-		} else if !isNullish(tmp) {
+		if !isUndefined || !isNullish(tmp) {
 			results[argDef.PrivateName] = tmp
 		}
 	}
 	return results
 }
 
+// Returns true if value is a reference to a variable the caller did not supply.
+func isUnprovidedVariable(value ast.Value, variables map[string]interface{}) bool {
+	v, ok := value.(*ast.Variable)
+	if !ok || v.Name == nil {
+		return false
+	}
+	_, provided := variables[v.Name.Value]
+	return !provided
+}
+
 // Given a variable definition, and any value of input, return a value which
 // adheres to the variable definition, or throw an error.
 func getVariableValue(schema Schema, definitionAST *ast.VariableDefinition, input interface{}) (interface{}, error) {
@@ -381,16 +389,12 @@ func valueFromAST(valueAST ast.Value, ttype Input, variables map[string]interfac
 		}
 		return append(values, valueFromAST(valueAST, ttype.OfType, variables))
 	case *InputObject:
-		var (
-			ok bool
-			ov *ast.ObjectValue
-			of *ast.ObjectField
-		)
-		if ov, ok = valueAST.(*ast.ObjectValue); !ok {
+		ov, ok := valueAST.(*ast.ObjectValue)
+		if !ok {
 			return nil
 		}
 		fieldASTs := map[string]*ast.ObjectField{}
-		for _, of = range ov.Fields {
+		for _, of := range ov.Fields {
 			if of == nil || of.Name == nil {
 				continue
 			}
@@ -398,20 +402,12 @@ func valueFromAST(valueAST ast.Value, ttype Input, variables map[string]interfac
 		}
 		obj := map[string]interface{}{}
 		for name, field := range ttype.Fields() {
-			var (
-				value       interface{}
-				isUndefined bool
-			)
-			if of, ok = fieldASTs[name]; ok {
-				value = valueFromAST(of.Value, field.Type, variables)
-			} else {
-				isUndefined = true
-				value = field.DefaultValue
-			}
-			if !isUndefined && value == nil {
-				obj[name] = nil
-			} else if !isNullish(value) {
-				obj[name] = value
+			of, ok := fieldASTs[name]
+			supplied := ok && !isUnprovidedVariable(of.Value, variables)
+			if supplied {
+				obj[name] = valueFromAST(of.Value, field.Type, variables)
+			} else if !isNullish(field.DefaultValue) {
+				obj[name] = field.DefaultValue
 			}
 		}
 		return obj