glDrawArrays, seems to ignore stride parameter?

The following works, to draw a square:
		float a[] = {
				   	  -1, 1, 0,
			          1, 1, 0,
					  -1,-1, 0,
					  1,1,0,
					  -1,-1,0,
					  1,-1,0,
		};
		glVertexPointer( 3, GL_FLOAT, 0, a );
		glDrawArrays( GL_TRIANGLES, 0, 6 );

The following code has the same logic, but I have inserted an extra 
float between each vertex.  I tell opengl by setting the stride to be 
sizeof(float).  But the following make square scrambled.
		float a[] = {
				   	  -1, 1, 0,
			99,
			          1, 1, 0,
			99,
					  -1,-1, 0,
			99,
					  1,1,0,
			99,
					  -1,-1,0,
			99,
					  1,-1,0,
			99,
		};
		glVertexPointer( 3, GL_FLOAT, sizeof(float), a );
		glDrawArrays( GL_TRIANGLES, 0, 6 );

Thanks,
~S