diff --git a/size.go b/size.go
index 44616c2..23cdd6b 100644
--- a/size.go
+++ b/size.go
@@ -26,7 +26,7 @@ const (
 	PiB = 1024 * TiB
 )
 
-type unitMap map[string]int64
+type unitMap map[string]uint64
 
 var (
 	decimalMap = unitMap{"k": KB, "m": MB, "g": GB, "t": TB, "p": PB}
@@ -75,28 +75,29 @@ func BytesSize(size float64) string {
 
 // FromHumanSize returns an integer from a human-readable specification of a
 // size using SI standard (eg. "44kB", "17MB").
-func FromHumanSize(size string) (int64, error) {
+func FromHumanSize(size string) (uint64, error) {
 	return parseSize(size, decimalMap)
 }
 
 // RAMInBytes parses a human-readable string representing an amount of RAM
 // in bytes, kibibytes, mebibytes, gibibytes, or tebibytes and
-// returns the number of bytes, or -1 if the string is unparseable.
+// returns the number of bytes, or 0 and corresponding error if the string
+// is unparseable.
 // Units are case-insensitive, and the 'b' suffix is optional.
-func RAMInBytes(size string) (int64, error) {
+func RAMInBytes(size string) (uint64, error) {
 	return parseSize(size, binaryMap)
 }
 
 // Parses the human-readable size string into the amount it represents.
-func parseSize(sizeStr string, uMap unitMap) (int64, error) {
+func parseSize(sizeStr string, uMap unitMap) (uint64, error) {
 	matches := sizeRegex.FindStringSubmatch(sizeStr)
 	if len(matches) != 4 {
-		return -1, fmt.Errorf("invalid size: '%s'", sizeStr)
+		return 0, fmt.Errorf("invalid size: '%s'", sizeStr)
 	}
 
 	size, err := strconv.ParseFloat(matches[1], 64)
 	if err != nil {
-		return -1, err
+		return 0, err
 	}
 
 	unitPrefix := strings.ToLower(matches[3])
@@ -104,5 +105,5 @@ func parseSize(sizeStr string, uMap unitMap) (int64, error) {
 		size *= float64(mul)
 	}
 
-	return int64(size), nil
+	return uint64(size), nil
 }
diff --git a/size_test.go b/size_test.go
index 8923e50..cf9a44b 100644
--- a/size_test.go
+++ b/size_test.go
@@ -125,7 +125,7 @@ func TestRAMInBytes(t *testing.T) {
 
 	assertSuccessEquals(t, 32, RAMInBytes, "32.3")
 	tmp := 32.3 * MiB
-	assertSuccessEquals(t, int64(tmp), RAMInBytes, "32.3 mb")
+	assertSuccessEquals(t, uint64(tmp), RAMInBytes, "32.3 mb")
 
 	assertError(t, RAMInBytes, "")
 	assertError(t, RAMInBytes, "hello")
@@ -142,7 +142,7 @@ func assertEquals(t *testing.T, expected, actual interface{}) {
 }
 
 // func that maps to the parse function signatures as testing abstraction
-type parseFn func(string) (int64, error)
+type parseFn func(string) (uint64, error)
 
 // Define 'String()' for pretty-print
 func (fn parseFn) String() string {
@@ -150,7 +150,7 @@ func (fn parseFn) String() string {
 	return fnName[strings.LastIndex(fnName, ".")+1:]
 }
 
-func assertSuccessEquals(t *testing.T, expected int64, fn parseFn, arg string) {
+func assertSuccessEquals(t *testing.T, expected uint64, fn parseFn, arg string) {
 	res, err := fn(arg)
 	if err != nil || res != expected {
 		t.Errorf("%s(\"%s\") -> expected '%d' but got '%d' with error '%v'", fn, arg, expected, res, err)
@@ -159,7 +159,7 @@ func assertSuccessEquals(t *testing.T, expected int64, fn parseFn, arg string) {
 
 func assertError(t *testing.T, fn parseFn, arg string) {
 	res, err := fn(arg)
-	if err == nil && res != -1 {
+	if err == nil && res != 0 {
 		t.Errorf("%s(\"%s\") -> expected error but got '%d'", fn, arg, res)
 	}
 }