diff --git a/R/guide-colorbar.R b/R/guide-colorbar.R
index 2710860333..c38a5fc2d2 100644
--- a/R/guide-colorbar.R
+++ b/R/guide-colorbar.R
@@ -291,10 +291,10 @@ GuideColourbar <- ggproto(
     # We set the defaults in `theme` so that the `params$theme` can still
     # overrule defaults given here
     if (params$direction == "horizontal") {
-      theme$legend.key.width <- rel(5)
+      theme$legend.key.width <- (theme$legend.key.width %||% rel(1)) * 5
       valid_position <- c("bottom", "top")
     } else {
-      theme$legend.key.height <- rel(5)
+      theme$legend.key.height <- (theme$legend.key.height %||% rel(1)) * 5
       valid_position <- c("right", "left")
     }
 
diff --git a/R/guides-.R b/R/guides-.R
index 38a94252b0..4c208506c0 100644
--- a/R/guides-.R
+++ b/R/guides-.R
@@ -941,7 +941,11 @@ redistribute_null_units <- function(units, spacing, margin, type = "width") {
 
   # Get spacing between guides and margins in absolute units
   size    <- switch(type, width = width_cm, height = height_cm)
-  spacing <- sum(rep(spacing, length.out = length(units) - 1))
+  if (length(units) < 2) {
+    spacing <- unit(0, "cm")
+  } else {
+    spacing <- sum(rep(spacing, length.out = length(units) - 1))
+  }
   margin  <- switch(type, width = margin[c(2, 4)], height = margin[c(1, 3)])
   margin  <- sum(size(margin))