diff --git a/src/blitter/common.hpp b/src/blitter/common.hpp
index 6b36feb980..0347aa8002 100644
--- a/src/blitter/common.hpp
+++ b/src/blitter/common.hpp
@@ -90,13 +90,14 @@ void Blitter::DrawLineGeneric(int x1, int y1, int x2, int y2, int screen_width,
 		if (x1 < 0) {
 			dash_count = (-x1) % (dash + gap);
 			auto adjust_frac = [&](int64_t frac, int &y_bound) -> int {
-				frac -= ((int64_t) dy) * ((int64_t) x1);
+				frac -= ((int64_t) dy) * ((int64_t) (x1 + 1));
 				if (frac >= 0) {
 					int quotient = frac / dx;
 					int remainder = frac % dx;
 					y_bound += (1 + quotient) * stepy;
 					frac = remainder - dx;
 				}
+				frac += dy;
 				return frac;
 			};
 			frac_low = adjust_frac(frac_low, y_low);
@@ -152,13 +153,14 @@ void Blitter::DrawLineGeneric(int x1, int y1, int x2, int y2, int screen_width,
 		if (y1 < 0) {
 			dash_count = (-y1) % (dash + gap);
 			auto adjust_frac = [&](int64_t frac, int &x_bound) -> int {
-				frac -= ((int64_t) dx) * ((int64_t) y1);
+				frac -= ((int64_t) dx) * ((int64_t) (y1 + 1));
 				if (frac >= 0) {
 					int quotient = frac / dy;
 					int remainder = frac % dy;
 					x_bound += (1 + quotient) * stepx;
 					frac = remainder - dy;
 				}
+				frac += dx;
 				return frac;
 			};
 			frac_low = adjust_frac(frac_low, x_low);