diff --git a/src/cpp-utils/data/DataFixture.cpp b/src/cpp-utils/data/DataFixture.cpp
index 4707bffe..e534fde6 100644
--- a/src/cpp-utils/data/DataFixture.cpp
+++ b/src/cpp-utils/data/DataFixture.cpp
@@ -2,22 +2,22 @@
 #include "SerializationHelper.h"
 
 namespace cpputils {
-  Data DataFixture::generate(size_t size, long long int seed) {
+  Data DataFixture::generate(size_t size, unsigned long long int seed) {
     Data result(size);
-    long long int val = seed;
-    for(size_t i=0; i<size/sizeof(long long int); ++i) {
+    unsigned long long int val = seed;
+    for(size_t i=0; i<size/sizeof(unsigned long long int); ++i) {
       //MMIX linear congruential generator
       val *= 6364136223846793005L;
       val += 1442695040888963407;
-      serialize<long long int>(result.dataOffset(i*sizeof(long long int)), val);
+      serialize<unsigned long long int>(result.dataOffset(i*sizeof(unsigned long long int)), val);
     }
-    uint64_t alreadyWritten = (size/sizeof(long long int))*sizeof(long long int);
+    uint64_t alreadyWritten = (size/sizeof(unsigned long long int))*sizeof(unsigned long long int);
     val *= 6364136223846793005L;
     val += 1442695040888963407;
-    char *remainingBytes = reinterpret_cast<char*>(&val);
+    unsigned char *remainingBytes = reinterpret_cast<unsigned char*>(&val);
     //Fill remaining bytes
-    for(size_t i=0; i<size-alreadyWritten; ++i) {
-      serialize<char>(result.dataOffset(alreadyWritten + i), remainingBytes[i]);
+    for(size_t i=0; i<sizeof(unsigned long long int); ++i) {
+      serialize<unsigned char>(result.dataOffset(alreadyWritten + i), remainingBytes[i]);
     }
     return result;
   }
diff --git a/src/cpp-utils/data/DataFixture.h b/src/cpp-utils/data/DataFixture.h
index a2ac705f..00fac978 100644
--- a/src/cpp-utils/data/DataFixture.h
+++ b/src/cpp-utils/data/DataFixture.h
@@ -9,7 +9,7 @@ namespace cpputils {
 
 class DataFixture final {
 public:
-  static Data generate(size_t size, long long int seed = 1);
+  static Data generate(size_t size, unsigned long long int seed = 1);
 
   //TODO Test
   template<size_t SIZE> static FixedSizeData<SIZE> generateFixedSize(long long int seed = 1);