diff --git a/src/libc/isdigit.src b/src/libc/isdigit.src
index f42d724e9..d787e56af 100644
--- a/src/libc/isdigit.src
+++ b/src/libc/isdigit.src
@@ -8,7 +8,6 @@ _isdigit:
 	push	de
 	ld	a,l
 	sub	a,48
-	add	a,-10
+	sub	a,10
 	sbc	hl,hl
-	inc	hl
 	ret
diff --git a/src/libc/isgraph.src b/src/libc/isgraph.src
index f7db268e1..f805788b2 100644
--- a/src/libc/isgraph.src
+++ b/src/libc/isgraph.src
@@ -8,7 +8,6 @@ _isgraph:
 	push	de
 	ld	a,l
 	sub	a,33
-	add	a,-94
+	sub	a,94
 	sbc	hl,hl
-	inc	hl
 	ret
diff --git a/src/libc/islower.src b/src/libc/islower.src
index 4fa9381d0..f0f84aa9c 100644
--- a/src/libc/islower.src
+++ b/src/libc/islower.src
@@ -8,7 +8,6 @@ _islower:
 	push	de
 	ld	a,l
 	sub	a,97
-	add	a,-26
+	sub	a,26
 	sbc	hl,hl
-	inc	hl
 	ret
diff --git a/src/libc/isupper.src b/src/libc/isupper.src
index 5cf7197f8..36a68c81d 100644
--- a/src/libc/isupper.src
+++ b/src/libc/isupper.src
@@ -8,7 +8,6 @@ _isupper:
 	push	de
 	ld	a,l
 	sub	a,65
-	add	a,-26
+	sub	a,26
 	sbc	hl,hl
-	inc	hl
 	ret