diff --git a/DESCRIPTION b/DESCRIPTION
index aead763dd..8f12403ad 100644
--- a/DESCRIPTION
+++ b/DESCRIPTION
@@ -1,7 +1,7 @@
 Type: Package
 Package: see
 Title: Model Visualisation Toolbox for 'easystats' and 'ggplot2'
-Version: 0.9.0
+Version: 0.9.0.1
 Authors@R:
     c(person(given = "Daniel",
              family = "Lüdecke",
diff --git a/NEWS.md b/NEWS.md
index bf61ca065..d0c299923 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -1,3 +1,10 @@
+# see (development)
+
+## Changes
+
+- `plot()` for `p_function()` now checks the values of the `size_length`
+  argument, to give an informative error message when the input is not valid.
+
 # see 0.9.0
 
 ## Changes
diff --git a/R/plot.p_function.R b/R/plot.p_function.R
index e6f4e3a5e..878fe08d1 100644
--- a/R/plot.p_function.R
+++ b/R/plot.p_function.R
@@ -55,6 +55,12 @@ plot.see_p_function <- function(x,
   # make sure group is factor
   data_ci_segments$group <- as.factor(data_ci_segments$group)
 
+  # sanity check - size_line must be of length two, when we have more than
+  # one group (i.e. when we emphasize CI lines)
+  if (length(size_line) != 2 && insight::n_unique(data_ci_segments$group) == 2) {
+    insight::format_error("Length of `size_line` must of length 2, to match regular and emphasized interval lines.") # nolint
+  }
+
   # setup - no color/fill aes for ribbons when we have no facets
   if (!is.null(n_columns) || insight::n_unique(data_ribbon$Parameter) == 1) {
     p <- ggplot2::ggplot() +
diff --git a/vignettes/parameters.Rmd b/vignettes/parameters.Rmd
index aea10eb81..ad1c8e545 100644
--- a/vignettes/parameters.Rmd
+++ b/vignettes/parameters.Rmd
@@ -426,7 +426,7 @@ result <- p_function(model)
 plot(result)
 
 result <- p_function(model, ci_levels = c(0.5, 0.7, emph = 0.9))
-plot(result, size_line = 0.6, 1.0)
+plot(result, size_line = c(0.6, 1.2))
 ```
 
 ## Principal Component Analysis