diff --git a/configure.ac b/configure.ac
index b155ab7..40a526a 100644
--- a/configure.ac
+++ b/configure.ac
@@ -89,7 +89,7 @@ if test "x$enable_curl" = "xyes" ; then
 		[ AC_MSG_WARN([required header file curl/curl.h not found, libcurl will be disabled (see --help)]) ])
 fi
 
-AC_CHECK_HEADERS([fcntl.h curses.h term.h ncurses/term.h])
+AC_CHECK_HEADERS([fcntl.h curses.h term.h ncurses/term.h math.h])
 
 if test "$enable_termlib" = "yes" ; then
 
diff --git a/man/jp2a.1 b/man/jp2a.1
index 3913f1f..a7d1e5f 100644
--- a/man/jp2a.1
+++ b/man/jp2a.1
@@ -1,4 +1,4 @@
-.TH jp2a 1  "April 5, 2020" "version 1.1.0" "USER COMMANDS"
+.TH jp2a 1  "August 1, 2020" "version 1.1.0" "USER COMMANDS"
 .SH NAME
 jp2a \- convert JPEG images to ASCII
 .SH SYNOPSIS
@@ -117,7 +117,6 @@ a display with light characters on a dark background, you should invert the imag
 .TP
 .BI \-\-blue= ...
 When converting from RGB to grayscale, use the given weights to calculate luminance.
-These three floating point values must add up to exactly 1.0.
 The default is red=0.2989, green=0.5866 and blue=0.1145.
 .TP
 .BI \-\-size= WIDTHxHEIGHT
diff --git a/src/options.c b/src/options.c
index 71cf776..6fbaafb 100644
--- a/src/options.c
+++ b/src/options.c
@@ -26,6 +26,8 @@
 #include <term.h>
 #endif
 
+#include <math.h>
+
 #include "jp2a.h"
 #include "options.h"
 #include "terminal.h"
@@ -172,8 +174,12 @@ void help() {
 	fprintf(stderr, "Report bugs to <%s>\n", PACKAGE_BUGREPORT);
 }
 
-void precalc_rgb(const float red, const float green, const float blue) {
+void precalc_rgb(float red, float green, float blue) {
 	int n;
+	float sum = red + green + blue;
+	red /= sum;
+	green /= sum;
+	blue /= sum;
 	for ( n=0; n<256; ++n ) {
 		RED[n]   = ((float) n) * red / 255.0f;
 		GREEN[n] = ((float) n) * green / 255.0f;
@@ -369,8 +375,17 @@ void parse_options(int argc, char** argv) {
 		exit(1);
 	}
 
-	if ( (int)((redweight + greenweight + blueweight)*10000000.0f) != 10000000 ) {
-		fputs("Weights RED + GREEN + BLUE must equal 1.0\n", stderr);
+	if ( redweight < 0 || greenweight < 0 || blueweight < 0 ) {
+		fputs("Weights can't be negative.\n", stderr);
+		exit(1);
+	}
+	if ( !isfinite(redweight) || !isfinite(greenweight) || !isfinite(blueweight) ) {
+		// This can happen if a number can not be represented as floating point, e.g. 3e400.
+		fputs("Did not understand a weight - possibly to large.\n", stderr);
+		exit(1);
+	}
+	if ( (redweight + greenweight + blueweight) == 0.0 ) {
+		fputs("At least one weight must be non-zero.\n", stderr);
 		exit(1);
 	}