diff --git a/src/modules/graphics/opengl/ParticleSystem.cpp b/src/modules/graphics/opengl/ParticleSystem.cpp
index 061ee1657..4e069507f 100644
--- a/src/modules/graphics/opengl/ParticleSystem.cpp
+++ b/src/modules/graphics/opengl/ParticleSystem.cpp
@@ -493,7 +493,7 @@ namespace opengl
 				size_t i = (size_t)s;
 				size_t k = (i == sizes.size() - 1) ? i : i + 1; // boundary check (prevents failing on t = 1.0f)
 				s -= (float)i; // transpose s to be in interval [0:1]: i <= s < i + 1 ~> 0 <= s < 1
-				p->size = sizes[i] * (1.0f - t) + sizes[k] * t;
+				p->size = sizes[i] * (1.0f - s) + sizes[k] * s;
 
 				// Update color according to given intervals (as above)
 				s = t * (float)(colors.size() - 1);
diff --git a/src/modules/graphics/opengl/wrap_ParticleSystem.cpp b/src/modules/graphics/opengl/wrap_ParticleSystem.cpp
index 11086e5df..e757bfdb5 100644
--- a/src/modules/graphics/opengl/wrap_ParticleSystem.cpp
+++ b/src/modules/graphics/opengl/wrap_ParticleSystem.cpp
@@ -188,7 +188,7 @@ namespace opengl
 		ParticleSystem * t = luax_checkparticlesystem(L, 1);
 		float arg1 = (float)luaL_checknumber(L, 2);
 		float arg2 = (float)luaL_optnumber(L, 3, arg1);
-		float arg3 = (float)luaL_optnumber(L, 3, 0);
+		float arg3 = (float)luaL_optnumber(L, 4, 0);
 		t->setSpin(arg1, arg2, arg3);
 		return 0;
 	}