diff --git a/kwant/linalg/mumps.py b/kwant/linalg/mumps.py
index ad44a124372ac933f2993e80e90b4fdb540f65aa..8f4be581f2eb41762e2fd6969b6a890939cdbc06 100644
--- a/kwant/linalg/mumps.py
+++ b/kwant/linalg/mumps.py
@@ -301,14 +301,11 @@ class MUMPSContext(object):
         if reuse_analysis:
             if self.mumps_instance is None:
                 warnings.warn("Missing analysis although reuse_analysis=True. "
-                              "New analysis is performed.",
-                              RuntimeWarning)
-
+                              "New analysis is performed.", RuntimeWarning)
                 self.analyze(a, ordering=ordering, overwrite_a=overwrite_a)
             else:
                 dtype, row, col, data = _make_assembled_from_coo(a,
                                                                  overwrite_a)
-
                 if self.dtype != dtype:
                     raise ValueError("MUMPSContext dtype and matrix dtype "
                                      "incompatible!")
diff --git a/kwant/plotter.py b/kwant/plotter.py
index 85d10d9d82e97aeb21b7825be32b00cdc830a82f..db915f8fb15699b52aad45b928fef7d0fcbf5047 100644
--- a/kwant/plotter.py
+++ b/kwant/plotter.py
@@ -32,10 +32,10 @@ try:
     try:
         from mpl_toolkits import mplot3d
     except ImportError:
-        warnings.warn("3D plotting not available.")
+        warnings.warn("3D plotting not available.", RuntimeWarning)
 except ImportError:
     warnings.warn("matplotlib is not available, only iterator-providing"
-                  "functions will work.")
+                  "functions will work.", RuntimeWarning)
     _mpl_enabled = False
 
 from . import system, builder