diff --git a/modules/uncertainty/NEWS.md b/modules/uncertainty/NEWS.md
index ac0c0ee6484..869f9780b03 100644
--- a/modules/uncertainty/NEWS.md
+++ b/modules/uncertainty/NEWS.md
@@ -1,5 +1,10 @@
 # PEcAn.uncertainty 1.9.0.9000
 
+## Fixed
+
+* `flux.uncertainty()`: removed a duplicate assignment of `E2` (the same line appeared twice in sequence with no effect except wasted computation).
+* `flux.uncertainty()`: fixed a crash that occurred when all positive-flux magnitude bins contained `NA` error estimates. In that case the positive-slope model `mp` was never assigned, but the `else` branch for the negative-slope fallback unconditionally referenced `mp$coefficients[1]`, producing an "object 'mp' not found" error. The fallback now uses `else if (exists("mp", inherits = FALSE))` so it only runs when `mp` was actually fitted.
+
 * Multiple bugfixes in `input.ens.gen()` handling of parent ids (#3783):
     - No longer skips inputs that have a parent but no sampling method.
     - Argument `parent_ids` now accepts integer vectors even if not wrapped in a list.
diff --git a/modules/uncertainty/R/flux_uncertainty.R b/modules/uncertainty/R/flux_uncertainty.R
index ef18e1ad31d..65bf39e6164 100644
--- a/modules/uncertainty/R/flux_uncertainty.R
+++ b/modules/uncertainty/R/flux_uncertainty.R
@@ -105,23 +105,22 @@ flux.uncertainty <- function(measurement, QC = 0, flags = TRUE, bin.num = 10,
   ## would be better to fit a two line model with a common intercept, but this
   ## is quicker to implement for the time being
   E2 <- errBin - errBin[zero]
-  E2 <- errBin - errBin[zero]
   intercept <- errBin[zero]
-  
-  return.list <- list(mag = magBin, 
-                      err = errBin, 
-                      bias = biasBin, 
+
+  return.list <- list(mag = magBin,
+                      err = errBin,
+                      bias = biasBin,
                       n = nBin,
                       intercept = intercept)
-  
-  if(!all(is.na(E2[pos]))){
+
+  if (!all(is.na(E2[pos]))) {
     mp <- stats::lm(E2[pos] ~ magBin[pos] - 1)
     return.list$slopeP <- mp$coefficients[1]
-  } 
-  if(!all(is.na(E2[neg]))){
+  }
+  if (!all(is.na(E2[neg]))) {
     mn <- stats::lm(E2[neg] ~ magBin[neg] - 1)
     return.list$slopeN <- mn$coefficients[1]
-  }else{
+  } else if (exists("mp", inherits = FALSE)) {
     return.list$slopeN <- mp$coefficients[1]
   }
   
diff --git a/modules/uncertainty/tests/testthat/test.flux_uncertainty.R b/modules/uncertainty/tests/testthat/test.flux_uncertainty.R
new file mode 100644
index 00000000000..d8f96965434
--- /dev/null
+++ b/modules/uncertainty/tests/testthat/test.flux_uncertainty.R
@@ -0,0 +1,78 @@
+## Tests for flux.uncertainty()
+##
+## The bug fixed in this PR:
+##   When all flux measurements are negative (all bins fall below zero),
+##   the positive-flux linear model `mp` is never assigned.  The original
+##   code unconditionally referenced `mp` in the else-branch for slopeN,
+##   causing an "object 'mp' not found" error at runtime.
+##
+##   Fix: the else-branch is now `else if (exists("mp", inherits = FALSE))`
+##   so slopeN falls back to the positive model only when it was actually fit.
+
+## Helper: create paired flux measurements where even-indexed values
+## (which become the "magnitude" in flux.uncertainty) span a requested range.
+make_flux_series <- function(range_lo, range_hi, n_pairs = 80, sd_noise = 0.2, seed = 1) {
+  set.seed(seed)
+  base <- seq(range_lo, range_hi, length.out = n_pairs)
+  # interleave: odd positions = one reading, even positions = paired reading
+  meas <- as.vector(rbind(
+    base + stats::rnorm(n_pairs, sd = sd_noise),
+    base + stats::rnorm(n_pairs, sd = sd_noise)
+  ))
+  QC <- rep(0L, length(meas))
+  list(measurement = meas, QC = QC)
+}
+
+test_that("flux.uncertainty returns cleanly when all fluxes are negative (mp never assigned)", {
+  d <- make_flux_series(range_lo = -20, range_hi = -2)
+
+  # Before the fix this would throw "object 'mp' not found"
+  result <- suppressMessages(
+    capture.output(
+      val <- flux.uncertainty(d$measurement, QC = d$QC, bin.num = 5, minBin = 3),
+      type = "output"
+    )
+  )
+
+  expect_type(val, "list")
+  expect_true(all(c("mag", "err", "bias", "n", "intercept") %in% names(val)),
+    label = "required list elements present")
+  # No positive bins -> slopeP should not be set
+  expect_false("slopeP" %in% names(val),
+    label = "slopeP absent when no positive-flux bins exist")
+})
+
+test_that("flux.uncertainty fits both slopes for mixed positive/negative flux", {
+  d <- make_flux_series(range_lo = -15, range_hi = 15)
+
+  capture.output(
+    val <- flux.uncertainty(d$measurement, QC = d$QC, bin.num = 8, minBin = 3),
+    type = "output"
+  )
+
+  expect_type(val, "list")
+  expect_true("slopeP" %in% names(val), label = "slopeP present for mixed data")
+  expect_true("slopeN" %in% names(val), label = "slopeN present for mixed data")
+  expect_true(is.numeric(val$slopeP) && length(val$slopeP) == 1)
+  expect_true(is.numeric(val$slopeN) && length(val$slopeN) == 1)
+})
+
+test_that("flux.uncertainty uses positive-slope fallback for slopeN when neg bins all NA", {
+  # Data spans a zero crossing but stays almost entirely positive.
+  # With bin.num=8 and minBin=5 the tiny negative portion falls below the
+  # minimum bin size, so errBin[neg] is NA for every negative bin.
+  # The else-if branch therefore assigns slopeN = mp$coefficients[1],
+  # making slopeN equal to slopeP.
+  d <- make_flux_series(range_lo = -0.5, range_hi = 20, seed = 3)
+
+  capture.output(
+    val <- flux.uncertainty(d$measurement, QC = d$QC, bin.num = 8, minBin = 5),
+    type = "output"
+  )
+
+  expect_type(val, "list")
+  expect_true("slopeP" %in% names(val), label = "slopeP present when positive bins have data")
+  expect_true("slopeN" %in% names(val), label = "slopeN set via mp fallback when neg bins empty")
+  expect_equal(val$slopeN, val$slopeP,
+    label = "slopeN equals slopeP when neg bins unavailable and mp is the fallback")
+})