diff --git a/CHANGELOG.md b/CHANGELOG.md
index eb135b3c..f171e9e2 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,10 +5,16 @@ Changelog of Git Changelog.
 ## Unreleased
 ### No issue
 
+**Checking for null in API-calls**
+
+ * For better error messages. 
+
+[18c745ae65282d7](https://github.com/tomasbjerre/violations-lib/commit/18c745ae65282d7) Tomas Bjerre *2017-08-11 09:42:01*
+
 **Testing with reporter**
 
 
-[b68214779e22b79](https://github.com/tomasbjerre/violations-lib/commit/b68214779e22b79) Tomas Bjerre *2017-07-14 19:30:01*
+[b73430f34a55ff6](https://github.com/tomasbjerre/violations-lib/commit/b73430f34a55ff6) Tomas Bjerre *2017-07-14 19:31:04*
 
 
 ## 1.29
diff --git a/src/main/java/se/bjurr/violations/lib/ViolationsReporterApi.java b/src/main/java/se/bjurr/violations/lib/ViolationsReporterApi.java
index c12181b0..27939505 100644
--- a/src/main/java/se/bjurr/violations/lib/ViolationsReporterApi.java
+++ b/src/main/java/se/bjurr/violations/lib/ViolationsReporterApi.java
@@ -2,6 +2,7 @@
 
 import static java.util.logging.Level.FINE;
 import static se.bjurr.violations.lib.reports.ReportsFinder.findAllReports;
+import static se.bjurr.violations.lib.util.Utils.checkNotNull;
 import static se.bjurr.violations.lib.util.Utils.setReporter;
 
 import java.io.File;
@@ -27,17 +28,17 @@ public static ViolationsReporterApi violationsReporterApi() {
   private ViolationsReporterApi() {}
 
   public ViolationsReporterApi findAll(Parser parser) {
-    this.parser = parser;
+    this.parser = checkNotNull(parser, "parser");
     return this;
   }
 
   public ViolationsReporterApi withReporter(String reporter) {
-    this.reporter = reporter;
+    this.reporter = checkNotNull(reporter, "reporter");
     return this;
   }
 
   public ViolationsReporterApi inFolder(String folder) {
-    startFile = new File(folder);
+    startFile = new File(checkNotNull(folder, "folder"));
     if (!startFile.exists()) {
       throw new RuntimeException(folder + " not found");
     }
@@ -45,15 +46,15 @@ public ViolationsReporterApi inFolder(String folder) {
   }
 
   public List<Violation> violations() {
-    List<File> includedFiles = findAllReports(startFile, pattern);
+    final List<File> includedFiles = findAllReports(startFile, pattern);
     if (LOG.isLoggable(FINE)) {
       LOG.log(FINE, "Found " + includedFiles.size() + " reports:");
-      for (File f : includedFiles) {
+      for (final File f : includedFiles) {
         LOG.log(FINE, f.getAbsolutePath());
       }
     }
-    List<Violation> foundViolations = parser.findViolations(includedFiles);
-    boolean reporterWasSupplied =
+    final List<Violation> foundViolations = parser.findViolations(includedFiles);
+    final boolean reporterWasSupplied =
         reporter != null && !reporter.trim().isEmpty() && !reporter.equals(parser.name());
     if (reporterWasSupplied) {
       setReporter(foundViolations, reporter);
@@ -61,7 +62,7 @@ public List<Violation> violations() {
 
     if (LOG.isLoggable(FINE)) {
       LOG.log(FINE, "Found " + foundViolations.size() + " violations:");
-      for (Violation v : foundViolations) {
+      for (final Violation v : foundViolations) {
         LOG.log(
             FINE,
             v.getReporter()