diff --git a/plotly/matplotlylib/mpltools.py b/plotly/matplotlylib/mpltools.py
index 0a3206998ba..4875c2babdb 100644
--- a/plotly/matplotlylib/mpltools.py
+++ b/plotly/matplotlylib/mpltools.py
@@ -120,10 +120,10 @@ def merge_color_and_opacity(color, opacity):
 
     rgb_tup = hex_to_rgb(color)
     if opacity is None:
-        return "rgb {}".format(rgb_tup)
+        return "rgb{}".format(rgb_tup)
 
     rgba_tup = rgb_tup + (opacity,)
-    return "rgba {}".format(rgba_tup)
+    return "rgba{}".format(rgba_tup)
 
 
 def convert_va(mpl_va):
diff --git a/plotly/matplotlylib/tests/test_renderer.py b/plotly/matplotlylib/tests/test_renderer.py
index 81b7c222714..726dd507d38 100644
--- a/plotly/matplotlylib/tests/test_renderer.py
+++ b/plotly/matplotlylib/tests/test_renderer.py
@@ -233,3 +233,14 @@ def test_semitransparent_axes_background_preserved():
     plotly_fig = tls.mpl_to_plotly(fig)
 
     assert plotly_fig.layout.plot_bgcolor == "rgba(26, 51, 76, 0.4)"
+
+
+def test_line_color_is_valid_plotly_color():
+    """Converted line colors are valid plotly color strings: plotly rejects
+    a space between 'rgba' and the opening parenthesis."""
+    fig, ax = plt.subplots()
+    ax.plot([0, 1], [0, 1], color="red")
+
+    plotly_fig = tls.mpl_to_plotly(fig)
+
+    assert plotly_fig.data[0].line.color == "rgba(255, 0, 0, 1)"