diff --git a/Makefile b/Makefile
index ae99f38..b0152f4 100644
--- a/Makefile
+++ b/Makefile
@@ -10,13 +10,14 @@ TESTS_SRC = tests
 TARGET = usage
 TEST_V_TARGET = test_vector
 TEST_M_TARGET = test_map
+TEST_B_TARGET = test_bigint
 
 LIB_OBJS = $(OBJ_DIR)/vector.o $(OBJ_DIR)/map.o $(OBJ_DIR)/bigint.o
 PROG_OBJS = $(OBJ_DIR)/usage.o
 
 .PHONY: all clean
 
-all: $(TARGET) $(TEST_V_TARGET) $(TEST_M_TARGET)
+all: $(TARGET) $(TEST_V_TARGET) $(TEST_M_TARGET) $(TEST_B_TARGET)
 
 $(TARGET): $(PROG_OBJS) $(LIB_OBJS)
 	$(CC) $(CFLAGS) -o $@ $^
@@ -27,6 +28,9 @@ $(TEST_V_TARGET): $(OBJ_DIR)/test_vector.o $(OBJ_DIR)/vector.o
 $(TEST_M_TARGET): $(OBJ_DIR)/test_map.o $(OBJ_DIR)/map.o
 	$(CC) $(CFLAGS) -o $@ $^
 
+$(TEST_B_TARGET): $(OBJ_DIR)/test_bigint.o $(OBJ_DIR)/bigint.o $(OBJ_DIR)/vector.o
+	$(CC) $(CFLAGS) -o $@ $^
+
 $(OBJ_DIR)/%.o: $(SRC_DIR)/%.c | $(OBJ_DIR)
 	$(CC) $(CFLAGS) -c -o $@ $<
 
@@ -40,4 +44,4 @@ $(OBJ_DIR):
 	mkdir -p $(OBJ_DIR)
 
 clean:
-	rm -rf $(OBJ_DIR) $(TARGET) $(TEST_V_TARGET) $(TEST_M_TARGET)
+	rm -rf $(OBJ_DIR) $(TARGET) $(TEST_V_TARGET) $(TEST_M_TARGET) $(TEST_B_TARGET)
diff --git a/src/bigint.c b/src/bigint.c
index 31069d2..89a042f 100644
--- a/src/bigint.c
+++ b/src/bigint.c
@@ -61,33 +61,34 @@ bigint_result_t bigint_from_int(long long value) {
     number->digits = vec_res.value.vector;
     number->is_negative = (value < 0);
 
-    if (value < 0) {
-        value = -value;
-    } else if (value == 0) {
+    // Discard the sign since we don't need it anymore
+    unsigned long long abs_val = value < 0 ? -(unsigned long long)value : (unsigned long long)value;
+
+    if(abs_val == 0) {
         int zero = 0;
         vector_result_t push_res = vector_push(number->digits, &zero);
         if (push_res.status != VECTOR_OK) {
             vector_destroy(number->digits);
             free(number);
             result.status = BIGINT_ERR_INVALID;
-            COPY_MSG(result, vec_res.message);
+            COPY_MSG(result, push_res.message);
 
             return result;
         }
     } else {
-        while (value > 0) {
-            int digit = value % BIGINT_BASE;
+        while (abs_val != 0) {
+            int digit = abs_val % BIGINT_BASE;
             vector_result_t push_res = vector_push(number->digits, &digit);
             if (push_res.status != VECTOR_OK) {
                 vector_destroy(number->digits);
                 free(number);
                 result.status = BIGINT_ERR_INVALID;
-                COPY_MSG(result, vec_res.message);
+                COPY_MSG(result, push_res.message);
 
                 return result;
             }
 
-            value /= BIGINT_BASE;
+            abs_val /= BIGINT_BASE;
         }
     }
 
diff --git a/tests/test_bigint.c b/tests/test_bigint.c
new file mode 100644
index 0000000..711a446
--- /dev/null
+++ b/tests/test_bigint.c
@@ -0,0 +1,63 @@
+/*
+ * Unit tests for BigInt data type
+*/
+
+#define TEST(NAME) do { \
+    printf("Running test_%s...", #NAME); \
+    test_##NAME(); \
+    printf(" PASSED\n"); \
+} while(0)
+
+#include <stdio.h>
+#include <string.h>
+#include <stdlib.h>
+#include <assert.h>
+
+#include "../src/bigint.h"
+
+static void bigint_eq(const bigint_t *number, const char *expected) {
+    bigint_result_t to_str = bigint_to_string(number);
+    assert(to_str.status == BIGINT_OK);
+    assert(!strcmp(to_str.value.string_num, expected));
+    
+    free(to_str.value.string_num);
+}
+
+// Test creating big integers from int
+void test_bigint_from_int() {
+    bigint_result_t res = bigint_from_int(0);
+    
+    assert(res.status == BIGINT_OK);
+    bigint_eq(res.value.number, "0");
+    bigint_destroy(res.value.number);
+
+    res = bigint_from_int(10);
+    assert(res.status == BIGINT_OK);
+    bigint_eq(res.value.number, "10");
+    bigint_destroy(res.value.number);
+
+    res = bigint_from_int(-12345678900LL);
+    assert(res.status == BIGINT_OK);
+    bigint_eq(res.value.number, "-12345678900");
+    bigint_destroy(res.value.number);
+}
+
+// Test creating big integers from string
+void test_bigint_from_string() {
+    bigint_result_t res = bigint_from_string("00000123");
+
+    assert(res.status == BIGINT_OK);
+    bigint_eq(res.value.number, "123");
+    bigint_destroy(res.value.number);
+}
+
+int main(void) {
+    printf("=== Running BigInt unit tests ===\n\n");
+
+    TEST(bigint_from_int);
+    TEST(bigint_from_string);
+
+    printf("\n=== All tests passed ===\n");
+
+    return 0;
+}
\ No newline at end of file