diff --git a/src/utilities/TypeInfo.ts b/src/utilities/TypeInfo.ts
index e72dfb01fb..ec8b985761 100644
--- a/src/utilities/TypeInfo.ts
+++ b/src/utilities/TypeInfo.ts
@@ -214,7 +214,7 @@ export class TypeInfo {
         const listType: unknown = getNullableType(this.getInputType());
         const itemType: unknown = isListType(listType)
           ? listType.ofType
-          : listType;
+          : undefined;
         // List positions never have a default value.
         this._defaultValueStack.push(undefined);
         this._inputTypeStack.push(isInputType(itemType) ? itemType : undefined);
diff --git a/src/utilities/__tests__/TypeInfo-test.ts b/src/utilities/__tests__/TypeInfo-test.ts
index 5c04458c51..5af6033500 100644
--- a/src/utilities/__tests__/TypeInfo-test.ts
+++ b/src/utilities/__tests__/TypeInfo-test.ts
@@ -390,10 +390,10 @@ describe('visitWithTypeInfo', () => {
       ['enter', 'ObjectField', null, '[String]'],
       ['enter', 'Name', 'stringListField', '[String]'],
       ['leave', 'Name', 'stringListField', '[String]'],
-      ['enter', 'ListValue', null, 'String'],
+      ['enter', 'ListValue', null, 'String' /* the item type, not list type */],
       ['enter', 'StringValue', null, 'String'],
       ['leave', 'StringValue', null, 'String'],
-      ['leave', 'ListValue', null, 'String'],
+      ['leave', 'ListValue', null, 'String' /* the item type, not list type */],
       ['leave', 'ObjectField', null, '[String]'],
       ['leave', 'ObjectValue', null, 'ComplexInput'],
     ]);
@@ -457,4 +457,104 @@ describe('visitWithTypeInfo', () => {
       ['leave', 'SelectionSet', null, 'Human', 'Human'],
     ]);
   });
+
+  it('supports traversals of object literals of custom scalars', () => {
+    const schema = buildSchema(`
+      scalar GeoPoint
+    `);
+    const ast = parseValue('{x: 4.0, y: 2.0}');
+    const scalarType = schema.getType('GeoPoint');
+    invariant(scalarType != null);
+
+    const typeInfo = new TypeInfo(schema, scalarType);
+
+    const visited: Array<any> = [];
+    visit(
+      ast,
+      visitWithTypeInfo(typeInfo, {
+        enter(node) {
+          const type = typeInfo.getInputType();
+          visited.push([
+            'enter',
+            node.kind,
+            node.kind === 'Name' ? node.value : null,
+            String(type),
+          ]);
+        },
+        leave(node) {
+          const type = typeInfo.getInputType();
+          visited.push([
+            'leave',
+            node.kind,
+            node.kind === 'Name' ? node.value : null,
+            String(type),
+          ]);
+        },
+      }),
+    );
+
+    expect(visited).to.deep.equal([
+      // Everything within ObjectValue should have type: undefined since the
+      // contents of custom scalars aren't part of GraphQL schema definitions.
+      ['enter', 'ObjectValue', null, 'GeoPoint'],
+      ['enter', 'ObjectField', null, 'undefined'],
+      ['enter', 'Name', 'x', 'undefined'],
+      ['leave', 'Name', 'x', 'undefined'],
+      ['enter', 'FloatValue', null, 'undefined'],
+      ['leave', 'FloatValue', null, 'undefined'],
+      ['leave', 'ObjectField', null, 'undefined'],
+      ['enter', 'ObjectField', null, 'undefined'],
+      ['enter', 'Name', 'y', 'undefined'],
+      ['leave', 'Name', 'y', 'undefined'],
+      ['enter', 'FloatValue', null, 'undefined'],
+      ['leave', 'FloatValue', null, 'undefined'],
+      ['leave', 'ObjectField', null, 'undefined'],
+      ['leave', 'ObjectValue', null, 'GeoPoint'],
+    ]);
+  });
+
+  it('supports traversals of list literals of custom scalars', () => {
+    const schema = buildSchema(`
+      scalar GeoPoint
+    `);
+    const ast = parseValue('[4.0, 2.0]');
+    const scalarType = schema.getType('GeoPoint');
+    invariant(scalarType != null);
+
+    const typeInfo = new TypeInfo(schema, scalarType);
+
+    const visited: Array<any> = [];
+    visit(
+      ast,
+      visitWithTypeInfo(typeInfo, {
+        enter(node) {
+          const type = typeInfo.getInputType();
+          visited.push([
+            'enter',
+            node.kind,
+            node.kind === 'Name' ? node.value : null,
+            String(type),
+          ]);
+        },
+        leave(node) {
+          const type = typeInfo.getInputType();
+          visited.push([
+            'leave',
+            node.kind,
+            node.kind === 'Name' ? node.value : null,
+            String(type),
+          ]);
+        },
+      }),
+    );
+
+    expect(visited).to.deep.equal([
+      ['enter', 'ListValue', null, 'undefined'],
+      ['enter', 'FloatValue', null, 'undefined'],
+      ['leave', 'FloatValue', null, 'undefined'],
+      ['enter', 'FloatValue', null, 'undefined'],
+      ['leave', 'FloatValue', null, 'undefined'],
+      ['leave', 'ListValue', null, 'undefined'],
+    ]);
+  });
 });
diff --git a/src/validation/__tests__/VariablesInAllowedPositionRule-test.ts b/src/validation/__tests__/VariablesInAllowedPositionRule-test.ts
index ec839d7497..3628bfef99 100644
--- a/src/validation/__tests__/VariablesInAllowedPositionRule-test.ts
+++ b/src/validation/__tests__/VariablesInAllowedPositionRule-test.ts
@@ -398,4 +398,81 @@ describe('Validates OneOf Input Objects', () => {
       },
     ]);
   });
+
+  it('Custom scalars as arg', () => {
+    expectValid(`
+      query Query($point: GeoPoint) {
+        dog {
+          distanceFrom(loc: $point)
+        }
+      }`);
+  });
+
+  it('Forbids using custom scalar as builtin arg', () => {
+    expectErrors(`
+      query Query($point: GeoPoint) {
+        dog {
+          isAtLocation(x: $point, y: 10)
+        }
+      }
+    `).toDeepEqual([
+      {
+        locations: [
+          {
+            column: 19,
+            line: 2,
+          },
+          {
+            column: 27,
+            line: 4,
+          },
+        ],
+        message:
+          'Variable "$point" of type "GeoPoint" used in position expecting type "Int".',
+      },
+    ]);
+  });
+
+  it('Forbids using builtin scalar as custom scalar arg', () => {
+    expectErrors(`
+      query Query($x: Float) {
+        dog {
+          distanceFrom(loc: $x)
+        }
+      }
+    `).toDeepEqual([
+      {
+        locations: [
+          {
+            column: 19,
+            line: 2,
+          },
+          {
+            column: 29,
+            line: 4,
+          },
+        ],
+        message:
+          'Variable "$x" of type "Float" used in position expecting type "GeoPoint".',
+      },
+    ]);
+  });
+
+  it('Allows using variables inside object literal in custom scalar', () => {
+    expectValid(`
+      query Query($x: Float) {
+        dog {
+          distanceFrom(loc: {x: $x, y: 10.0})
+        }
+      }`);
+  });
+
+  it('Allows using variables inside list literal in custom scalar', () => {
+    expectValid(`
+      query Query($x: Float) {
+        dog {
+          distanceFrom(loc: [$x, 10.0])
+        }
+      }`);
+  });
 });
diff --git a/src/validation/__tests__/harness.ts b/src/validation/__tests__/harness.ts
index ea4840341c..15e720fe80 100644
--- a/src/validation/__tests__/harness.ts
+++ b/src/validation/__tests__/harness.ts
@@ -33,6 +33,8 @@ export const testSchema: GraphQLSchema = buildSchema(`
     DOWN
   }
 
+  scalar GeoPoint
+
   type Dog implements Pet & Mammal & Canine {
     name(surname: Boolean): String
     nickname: String
@@ -41,6 +43,7 @@ export const testSchema: GraphQLSchema = buildSchema(`
     doesKnowCommand(dogCommand: DogCommand): Boolean
     isHouseTrained(atOtherHomes: Boolean = true): Boolean
     isAtLocation(x: Int, y: Int): Boolean
+    distanceFrom(loc: GeoPoint): Float
     mother: Dog
     father: Dog
   }