diff --git a/R/measurement_utils.R b/R/measurement_utils.R
index 26d1741..0b578a6 100644
--- a/R/measurement_utils.R
+++ b/R/measurement_utils.R
@@ -2,7 +2,7 @@
 plot.osem_measurements = function (x, ...) {
   oldpar = par()
   par(mar = c(2,4,1,1))
-  plot(value~createdAt, x, col = x$sensorId, xlab = NA, ylab = x$unit[1], ...)
+  plot(value~createdAt, x, col = factor(x$sensorId), xlab = NA, ylab = x$unit[1], ...)
   par(mar = oldpar$mar)
   invisible(x)
 }