diff --git a/R/guide-axis-theta.R b/R/guide-axis-theta.R
index af96a337b6..5183e802fc 100644
--- a/R/guide-axis-theta.R
+++ b/R/guide-axis-theta.R
@@ -104,8 +104,9 @@ GuideAxisTheta <- ggproto(
 
       # If the first and last positions are close together, we merge the
       # labels of these positions
-      ends_apart <- (key$theta[n] - key$theta[1]) %% (2 * pi)
-      if (n > 0 && ends_apart < 0.05 && !is.null(key$.label)) {
+      if (n > 1L &&
+        (key$theta[n] - key$theta[1]) %% (2 * pi) < 0.05 &&
+        !is.null(key$.label)) {
         if (is.expression(key$.label[[1]])) {
           combined <- substitute(
             paste(a, "/", b),
diff --git a/tests/testthat/test-guide-axis.R b/tests/testthat/test-guide-axis.R
index 5e2010d1c5..85e1477024 100644
--- a/tests/testthat/test-guide-axis.R
+++ b/tests/testthat/test-guide-axis.R
@@ -382,6 +382,13 @@ test_that("guide_axis_theta sets relative angle", {
   expect_doppelganger("guide_axis_theta with angle adapting to theta", p)
 })
 
+test_that("guide_axis_theta with only one axis key", {
+  p <- ggplot(data.frame(x = "a", y = 1:3), aes(x, y)) +
+    geom_point() +
+    coord_radial()
+  expect_identical(get_guide_data(p, "theta")$.label, "a")
+})
+
 test_that("guide_axis_theta can be used in cartesian coordinates", {
 
   p <- ggplot(mtcars, aes(disp, mpg)) +