diff --git a/lib/handler/TypeHandlerBoolean.ts b/lib/handler/TypeHandlerBoolean.ts
index 82c5c36..a7f51ea 100644
--- a/lib/handler/TypeHandlerBoolean.ts
+++ b/lib/handler/TypeHandlerBoolean.ts
@@ -27,8 +27,8 @@ export class TypeHandlerBoolean implements ITypeHandler {
   }
 
   public toRdf(value: any, { datatype, dataFactory }: IToRdfOptions): RDF.Literal {
-    return dataFactory.literal(value ? 'true' : 'false',
-      datatype || dataFactory.namedNode(TypeHandlerBoolean.TYPE));
+    return dataFactory!.literal(value ? 'true' : 'false',
+      datatype || dataFactory!.namedNode(TypeHandlerBoolean.TYPE));
   }
 
 }
diff --git a/lib/handler/TypeHandlerDate.ts b/lib/handler/TypeHandlerDate.ts
index 23f2e66..2744201 100644
--- a/lib/handler/TypeHandlerDate.ts
+++ b/lib/handler/TypeHandlerDate.ts
@@ -45,11 +45,11 @@ export class TypeHandlerDate implements ITypeHandler {
   }
 
   public toRdf(value: any, { datatype, dataFactory }: IToRdfOptions): RDF.Literal {
-    datatype = datatype || dataFactory.namedNode(TypeHandlerDate.TYPES[0]);
+    datatype = datatype || dataFactory!.namedNode(TypeHandlerDate.TYPES[0]);
 
     // Assume date values
     if (!(value instanceof Date)) {
-      return null;
+      return null!; // TODO: throw error in next breaking change
     }
     const date: Date = <Date> value;
 
@@ -73,7 +73,7 @@ export class TypeHandlerDate implements ITypeHandler {
     default:
       valueString = date.toISOString();
     }
-    return dataFactory.literal(valueString, datatype);
+    return dataFactory!.literal(valueString, datatype);
   }
 
 }
diff --git a/lib/handler/TypeHandlerNumberDouble.ts b/lib/handler/TypeHandlerNumberDouble.ts
index dd95003..d0e6c31 100644
--- a/lib/handler/TypeHandlerNumberDouble.ts
+++ b/lib/handler/TypeHandlerNumberDouble.ts
@@ -25,17 +25,17 @@ export class TypeHandlerNumberDouble implements ITypeHandler {
   }
 
   public toRdf(value: any, { datatype, dataFactory }: IToRdfOptions): RDF.Literal {
-    datatype = datatype || dataFactory.namedNode(TypeHandlerNumberDouble.TYPES[0]);
+    datatype = datatype || dataFactory!.namedNode(TypeHandlerNumberDouble.TYPES[0]);
     if (isNaN(value)) {
-      return dataFactory.literal('NaN', datatype);
+      return dataFactory!.literal('NaN', datatype);
     }
     if (!isFinite(value)) {
-      return dataFactory.literal(value > 0 ? 'INF' : '-INF', datatype);
+      return dataFactory!.literal(value > 0 ? 'INF' : '-INF', datatype);
     }
     if (value % 1 === 0) {
-      return null;
+      return null!; // TODO: throw error in next breaking change
     }
-    return dataFactory.literal(value.toExponential(15).replace(/(\d)0*e\+?/, '$1E'), datatype);
+    return dataFactory!.literal(value.toExponential(15).replace(/(\d)0*e\+?/, '$1E'), datatype);
   }
 
 }
diff --git a/lib/handler/TypeHandlerNumberInteger.ts b/lib/handler/TypeHandlerNumberInteger.ts
index daa9ae9..0ab82f5 100644
--- a/lib/handler/TypeHandlerNumberInteger.ts
+++ b/lib/handler/TypeHandlerNumberInteger.ts
@@ -37,10 +37,10 @@ export class TypeHandlerNumberInteger implements ITypeHandler {
   }
 
   public toRdf(value: any, { datatype, dataFactory }: IToRdfOptions): RDF.Literal {
-    return dataFactory.literal(String(value), datatype
+    return dataFactory!.literal(String(value), datatype
       || (value <= TypeHandlerNumberInteger.MAX_INT && value >= TypeHandlerNumberInteger.MIN_INT
-        ? dataFactory.namedNode(TypeHandlerNumberInteger.TYPES[0])
-        : dataFactory.namedNode(TypeHandlerNumberInteger.TYPES[1])));
+        ? dataFactory!.namedNode(TypeHandlerNumberInteger.TYPES[0])
+        : dataFactory!.namedNode(TypeHandlerNumberInteger.TYPES[1])));
   }
 
 }
diff --git a/lib/handler/TypeHandlerString.ts b/lib/handler/TypeHandlerString.ts
index e401bc4..f541d13 100644
--- a/lib/handler/TypeHandlerString.ts
+++ b/lib/handler/TypeHandlerString.ts
@@ -18,6 +18,7 @@ export class TypeHandlerString implements ITypeHandler {
     'http://www.w3.org/2001/XMLSchema#token',
     'http://www.w3.org/2001/XMLSchema#hexBinary',
     'http://www.w3.org/1999/02/22-rdf-syntax-ns#langString',
+    'http://www.w3.org/1999/02/22-rdf-syntax-ns#dirLangString',
 
     'http://www.w3.org/2001/XMLSchema#time',
     'http://www.w3.org/2001/XMLSchema#duration',
@@ -28,7 +29,7 @@ export class TypeHandlerString implements ITypeHandler {
   }
 
   public toRdf(value: any, { datatype, dataFactory }: IToRdfOptions): RDF.Literal {
-    return dataFactory.literal(value, datatype);
+    return dataFactory!.literal(value, datatype);
   }
 
 }
diff --git a/test/index-test.ts b/test/index-test.ts
index 0cc4506..610d9b6 100644
--- a/test/index-test.ts
+++ b/test/index-test.ts
@@ -17,6 +17,11 @@ describe('fromRdf', () => {
         .toEqual('abc');
     });
 
+    it('should handle language tagged strings with direction', () => {
+      return expect(fromRdf(DF.literal('abc', { language: 'en-us', direction: 'ltr' })))
+        .toEqual('abc');
+    });
+
     it('should handle normalized strings', () => {
       return expect(fromRdf(DF.literal('abc',
         DF.namedNode('http://www.w3.org/2001/XMLSchema#normalizedString'))))
@@ -655,6 +660,7 @@ describe('getSupportedRdfDatatypes', () => {
       DF.namedNode('http://www.w3.org/2001/XMLSchema#token'),
       DF.namedNode('http://www.w3.org/2001/XMLSchema#hexBinary'),
       DF.namedNode('http://www.w3.org/1999/02/22-rdf-syntax-ns#langString'),
+      DF.namedNode('http://www.w3.org/1999/02/22-rdf-syntax-ns#dirLangString'),
 
       DF.namedNode('http://www.w3.org/2001/XMLSchema#time'),
       DF.namedNode('http://www.w3.org/2001/XMLSchema#duration'),
diff --git a/tsconfig.json b/tsconfig.json
index ec158fd..93e1d47 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -14,7 +14,8 @@
     "sourceMap": true,
     "inlineSources": true,
     "declaration": true,
-    "importHelpers": false
+    "importHelpers": false,
+    "strict": true
   },
   "include": [
     "index.ts",