diff --git a/test/test_CreateCustomer.c b/test/test_CreateCustomer.c
index f681375..85122a7 100644
--- a/test/test_CreateCustomer.c
+++ b/test/test_CreateCustomer.c
@@ -2,6 +2,7 @@
 #include <limits.h>
 #include <math.h>
 #include "../src/stringManipulation.c"
+#include "../src/error.c"
 #include "../src/createCustomer.c"
 
 void setUp(){}
diff --git a/test/test_LoginCustomer.c b/test/test_LoginCustomer.c
index 0bff2d2..a293a0d 100644
--- a/test/test_LoginCustomer.c
+++ b/test/test_LoginCustomer.c
@@ -1,6 +1,7 @@
 #include <unity.h>
 #include "../src/loginCustomer.c"
 #include "../src/stringManipulation.c"
+#include "../src/error.c"
 void setUp(){};
 void tearDown(){};
 void test_checkLogin()
diff --git a/test/test_StringManipulation.c b/test/test_StringManipulation.c
index 3a23b57..efbd1b2 100644
--- a/test/test_StringManipulation.c
+++ b/test/test_StringManipulation.c
@@ -4,6 +4,36 @@
 #include <unity.h>
 #include "../src/stringManipulation.c"
 
+
+void test_isLetterOfAlphabet()
+{
+	/*test block 1*/
+	char *testStringsToTrue[] = {"adhj","kasdlwq","vbeuqor","kalkwyynmakj","kakswxl","akljlxjkcyxyklj","asdjhuwpwe","xbcvddd","klajksdjkl","ghghgtie","kajsd"};
+	unsigned int numberOfElements = sizeof(testStringsToTrue) / sizeof(char *);
+	for(int i=0;i<numberOfElements;++i){
+		TEST_ASSERT_TRUE(isLetterOfAlphabet(testStringsToTrue[i]));
+	}
+	/*test block 2*/
+	char *testStringsToTrue_2[] = {"bcjyxhkjyxchjqwo","tree","garden","thinker","philosophy","linux","computer","lesson","teacher","table","drink","water","every", "Frank","city","economic","programming","task","smart","smarter","smartest","dumb","wood","animals","forest","display","hot","cold","ice","bear","keyboard","pair","pencil"};
+	numberOfElements = sizeof(testStringsToTrue_2) / sizeof(char *);
+	for(int i=0;i<numberOfElements;++i){
+		TEST_ASSERT_TRUE(isLetterOfAlphabet(testStringsToTrue_2[i]));
+	}
+	/*test block 3*/
+	char *testStringsToFalse[] = {"ashjdkj32123","4213jashj","laskdj2","1sbabjsdh","askjasdkjd0","123932131a","abcd2hutz","81287asjk231jkhs","aslkjasdlkjsd123","cbc451873"};
+	numberOfElements = sizeof(testStringsToFalse) / sizeof(char *);
+	for(int i=0;i<numberOfElements;++i){
+		TEST_ASSERT_FALSE(isLetterOfAlphabet(testStringsToFalse[i]));
+	}
+	/*test block 4*/
+	char *testStringsToFalse_2[] = {"1234","56789","00000010101010","3748927398273498","757575757","1726371238726","19237182937192837","875378612873621","128973192837","99494949499929292929292938382828","1827391237981273","7481263871236782136"};
+	numberOfElements = sizeof(testStringsToFalse_2) / sizeof(char *);
+	for(int i=0;i<numberOfElements;++i){
+		TEST_ASSERT_FALSE(isLetterOfAlphabet(testStringsToFalse_2[i]));
+	}
+}
+
+
 void test_toUnsignedInteger()
 {	
 	/*test block 1*/